Anything; monsters, climate, scenery etc.
Personally, I would love to have more water based monsters. When I go diving for chests I always have a slight concern that something is going to grab me, especially in that spider cave, why is there no water spider lurking? :P
Has anybody played Lord of the Rings: The Two Towers on PS2? Do you remember fighting The Guardian of the West Gate? The tentacleish thing in the water? That kind of thing too!