Aspects of Boating
Boat Types and Technology Requirements
There are three kinds of boats (and the Portuguese Carrack) that can carry military units.
GalleyThe Galley has two moves and cannot enter ocean unless that is within the player´s (or his team´s) cultural borders. Only in Ancient start games it is not available from the start, but can be researched easily. The humble number of moves and the restriction to coast and ocean tiles within cultural borders rarely allows for surprise attacks, at least rarely such moves which couldn´t at least have been foreseen by the defender. At this point I recommend the article on Vision Range which covers sea unit and general sea vision ranges.
A special exception, the 5-move-galley:
In order to promote a galley with a general, it has to be empty and the general loaded in and then attached. Once done the galley has access to promotions Morale, Flanking I -> Navigation I and II, giving it an overall of 5 moves.
This can be used to hit opposing cities that otherwise at this technological point in the game wouldn´t be accessible by sea units or at least none with 5 moves. It´s not often a 5-move-galley can kill a city because of its suprising nature, but I´ve seen it done in competitive games more often than you´d think.
CarrackThe Carrack is the Portuguese Unique Unit, it is a Caravel that can carry two military units. There is no faster way of moving units over bigger parts of ocean then this ship. In can be used best in scenarios where boating otherwise wouldn´t be an option till Astronomy and with it Galeons is researched. It's biggest strength is probably not the actual act of sending units over to another continent/island, but the opponent´s fear of that happening. Someone who is afraid of getting attacked potentially plants cities different and gets (at this point unnecessary) units earlier instead of focusing solely on buildup.
GalleonWith Galeons boating really begins. They can carry a more significant amount of three units and have at least four moves (without promotions). From late Medieval till early Industrial they are the boating unit of choice. Getting Astronomy can be a strategy in itself in Medieval and Renaissance (and even earlier eras) in order to be able to attack at all (continents like maps) or in order to launch strikes at otherwise not accessible spots. The possibility of direct hitting a coast city or of landing units at many different spots puts the opponent in a much tougher position when having to defend (and position his units).
East IndiamanThe East Indiaman is the dutch Unique Unit, replacing the Galeon. With space for four units and a base strength of six instead of four this shines most in games where Astronomy isn´t available from the start. There it can not only transport 33% more units, but also has great odds on opposing Galeons.
TransportTransports are a level further than Galeons with more moves and space for four units. They are of course better, but don´t bring a completely new aspect to a scenario unlike Galeons. Though you of course rather want to be able to build them (uranium, oil!) and not Galeons in games where Combustion is available from the start (modern, future start).
Boat production: Investment and Gain
No matter at which point you want to transport units via boat - you have to build those ships first. This of course is stupidly obvious, but brings with it challenges. If you want to send the boats right at the beginning of an Industrial, Modern or Future game for example, you will have to delay some of your buildup. Instead of having a city producing workers for instance, it will be doing a boat or two. Instead of spending money on buying something else, you might be buying a Galeon or Transport. Later on if you want to boat a significant stack via Galeons for example, about every fourth unit you actually send over is a ship that won´t do any (meaningful) fighting. Additionally you might have to build cover ships as well, in order to not let your army be eaten by opposing sea units with much higher fire power than a Galeon or Transport in case your ship stack is spotted before reaching its target. This means that if your main goal is to get a stack from A (your land) to B (opponent´s land) a part of your production has to be channeled into boats instead of further buildup or more land units. As long as your boating doesn´t result in some strategic advantage like killing a city or at least more units than you lose yourself, you will have spent hammers in a potentially worse way then your opponent who might have been further developing a land-based stack.
Since Civ is a game that favours the defender already, sacrificing units in order to build unit carrying boats is only a good idea if you gain attack opportunities that allow you to endanger cities in a way that the opponent cannot use his entire forces to defend them. In general you can say that the bigger a map is the longer are the distances between cities and hence it is tougher to position a defending stack in a way that it can reach all crucial spots in the shortest possible time. On a map where you have only two land based fronts like Green_Ring, Inland_Sea or to a certain degree Green_Wheel boating can open up a much bigger front with the danger of units landing in the "back" while defending stacks have been positioned at the (land) front cities. On very open maps like Balanced or Pangae boating can be used to increase the amount of potentially endangered cities even more.
Ways of Boating
Direct Boating
Direct Boating is the act of hitting a city from sea with the units on board a unit carrying boat. The key aspect about this is the malus on Amphibious Attacks. If you hit a city directly from sea, the defending units receive a +50% bonus. This results in a ratio of units required to kill one defending unit of somewhere around 3:1 instead of the usual guideline of 2:1. This of course is only an option if you can get a city with relatively few units, ranging from one Transport or Galeon with 3 or 4 units hitting a barely planted city with one or two units in it to 5-6 boats loading 15-20 units. The last one is only an option for games with city elimination one or two. Otherwise it´s only worth the exchange of your hammers for the boats and units against the opponent´s hammers for the city and the units in it if it´s a crucial city in some way (tech city, GP city, wonder city, capital) since you will probably lose more production this way then your opponent.The main target for direct boating are cities with few defending units in it - may that be for whatever reason. So an opponent might not be expecting a boating because he oversaw the possibility for example. Also he might not be having the resources to secure all his fronts, possibly having to send everything to a land based front which is in danger of being hit (for example Future start on Balanced). There are many different reasons why a coast city can be defended weakly - realizing such opportunities and preparing the right measures in time demands skill and experience in multiplayer games.
Landing where defending stack cannot act fully and freely
A different possible way of attacking via sea is the boating of a bigger amount of units (10-12 earlier on, mid-game around 20-30) into a position where they can hit at least one, possibly multiple cities in the "back" land of a player or team. Especially on a bigger map with far apart land fronts defending stacks might be positioned far away from a back player - not having a way to move into every city in one turn in case of emergency. This way the boating player avoids the malus of Amphibious Attacks and possibly catches a back player with few units in his own land by surprise. It can easily happen that the attacking stack has the opportunity to hit more than one city the turn after having landed. In such a scenario it´s very likely that at least one city will fall.The tough thing about this way of boating is the fact that the opposing team will likely be guarding the sea with ships - which essentially is a front of its own for the reason of such boatings. Unless you are able to gain total sea control (pushing your opponent back to his own territory, guarding the sea with ship stacks) it is unlikely that your boats get through undetected. At best you get through but at least warn the opponent of your arrival by having to hit his sentry ships with own boats. If you get spotted the opponent can reposition his stack and you are back to fighting under "normal" conditions. If you land then, you will probably be attacked by a much more significant part of the opposing army.
Boating as the fastest way of reaching or doubling/tripling an opponent
This relates only to Industrial, Modern and Future start teamer games. There Galeons and/or Transport are available from the start. It can be the fastest way of getting units to an opponent to chop or buy a boat from turn 0ne and load in the starting units - and possibly another few chopped ones. If two players send one or two loaded boats to an opponent that just planted his initial settlers and is beginning buildup - possibly by getting workers and settler - it is possible that he won´t be able to defend alone against two players. This can even be true if only one players is doing the boating.Another version of this is the support of a teammate who is rushing on land towards an opponent. For example in Future start games mostly the first thing that happens around turns 4-6 is the planting and culture bombing of a city (by Gandhi of India) next to an opponent´s barely planted land. The starting units together with another 2-3 bought and chopped units build up huge pressure, very often resulting in at least one city kill. Another 3-4 units boated over by an opponent into that combat situation will most probably result in the final blow to the player being doubled at this super early point in the game.
Continents Teamer
In games where players start on different continents or islands than their opponents Astronomy is the first option of attacking someone. In team games it automatically also is the first option of doubling an opponent. Imagine an Ancient start 2v2 game where one side gets Astronomy a couple of turns earlier, builds Galeons and sends units from two players over to one opponent. The usual drawback of having less "fighting" units (boats+units are less units than defending player´s units only; also you do not/cannot send over your entire stack most of the time) is leveled out by being able to go 2v1 on one player. After killing him, only 1v1 defending on your own Island/Continent is required, while the sea can still be fought for together.The sheer strength lies in the fact that Galeons themselves are much stronger than any other ship available to an opponent before Astronomy and that they can transport units. They are essentially cover for themselves - especially if the Dutch East Indiaman is around. You know that if you get Astronomy before your opponent you WILL get through and be able to land with your boating if you put enough resources into it.
Resume
Boating is a great tool, especially in games with a city elimination limit. It rarely though can be used as a before the game planned out strategy. Most of the time opportunities for boating have to be detected and used on the spot when the opportunity arrives. When you play a game and consider boating - think about whether putting the additional hammers into boats instead of units does give you enough additional advantage that is worth the investment. In general the bigger the preferably cylindrical or toroidal map (with water on it of course) the more numerous the possible new attack options a boating might give you.
In the end boating is nothing else than weakening a part of your attack force in order to put them into a position where the defending units are even worse prepared for the task. Boating is about circumnavigating a big stack vs. big stack situation by finding another way of hitting the opponent.