I haven't even tried out multiplayer yet. To be completely honest, my PC just doesn't handle gaming very well. When it gets all laggy it feels like a chore trying to play, so i'm skipping it for now.
I agree with the underwater stuff, to quote X-play it was all about the 'Oooh, pretty!' It makes me wonder where they'll go with the next game, I don't know what else they can do without getting the 'been there, done that' feeling. Back when that first teaser came out some friends and I speculated they were hinting that Rapture was going to surface and you'd be on land for most of the game. Maybe that's a direction they can go in.