1st Use the sunken ruins terrain
2nd set the fog in map options to Linear, Z start 100, Z end 3500, and the fog color to a similar color to that of the water's color.
3rd if its a deep ocean then its best to go with a light blue fog and use the weather effect "Moonlight". If its a beach resort type of ocean its best to go with a teal range color of fog and water with "Rays of light" as the weather effect.
4th make a nice terrain using well varied doodads and nice tile set placement.
I hope this helps good luck.