FISH: Go into editor and enter the water with a diver. Shoot a fish and marvel as it stays in the exact same position under water. Also, maybe the fish could move out of the way or "flee" when approached by vehicles and soldiers?
BOATS: Go into editor and place either a rescue boat or assault boat in the water. Turn on the engine and start moving. You will see water effects coming up through the bottom and/or the sides/front of the boat.