Very fast movement on water, slow movement on land.
They get the powerful Drown skill, which allows them to deal double their Strength in damage to a single opponent, as long as they are on a water tile. If you want an aquatic battle unit a Shrimp is a good candidate.
Their vagina are a lot more lubricated than normal, so the effect works even underwater. This makes possible to insert even the biggest cocks with relative ease into their tight pussies.