This looks perfectly normal, underwater caustics have always been that way. Still looks better than just turning them off though.
At least they're not completelely broken like they are on the latest 9xx nvidia cards (
https://www.youtube.com/watch?v=pTg-D1_AY3I ), you should be happy :p
edit : oh wait you mean that large square area thingy, I am not sure but the game is full of graphical glitches anyway, do you see it everywhere? or just in a specific place? if so, where exactly was this taken? I'm thinking Paikang?