Let's come up with an amphibian race!

Heroes could be Nagas (like in Warcraft 3); their ability could be moving across the sea without a ship, which is already a big plus, but something else could be added, otherwise, in maps without the sea, they are practically without abilities.

Added 5 minutes later
If the castle is on the water, it will create a balance issue; it should just be surrounded by water, but still stand on land.