Attacking Aquatic Outposts.

[Dr Strangelove]

I'm trying to capture/destroy an enemy's aquatic outpost but the game prohibits me from making a melee attack. The outpost is situated on a coast hex. I've bombarded it's defenses to nothing, but when I move a marine next to it and try to attack the move is prohibited. I tried embarking the marine and then attacking, but same problem. So how does one finish off an aquatic outpost?
Also, once you've accumulated a massive advantage in war score, say 320 to 0, how long does it take the AI to accept reality and allow a parlay for peace?