Sea grean ground that is littered with seashells.
Maybe with bubbles floating about.
Instead of trees have coral.
Ruins could look like sunken ships.
Underwater bubbling noises in the background.
And later start offering oceanic based skins for troops and buildings.
Greenish blue skin.
Armor made from colorful shells.
The mummy would be wrapped in seaweed instead of rags.
The ogre could have fish eyes and ears that look like fins.
The knights and archers could resemble sea elves.
The werewolf in a pirate suit.
The towers would spiral and have shells on them.
The barricades would look like rusty gates.
That sort of stuff.