I don't like the walls, but I don't really know what else you could do, since there's not really a way to simulate an ocean and it looks bad to jush have it abruptly cut off by sky / floating in the void. One possibility might be to make it look like a scenario field -- like make it look like a real paintball field with various bunkers set up to reproduce a dday scenario. I saw one field that had tank traps made out of wood, for example, and it looked kind of cool. It might be hard to pull it off well, though.
Another thing -- get some lighting in there before you take any more screenshots.