First of all we need ways to add water:
Map redesign with more and broader areas of water, deep enough to trigger UW combat, doesn't need to be that deep, honestly don't need the 3d combat, just enough to swap skills.
Ruins/other way to raise the water level, either for a duration, or until deactivated.
A tactics to put in fortified structures, to flood the entire area inside the walls. Either repair behind them and activate to catch them in water (or they have to find a high ground or swim to walls) for a duration, or activate it before they breach the wall to have the water flood out as they break the wall and sweep them away (hillarious at hills wall!)
How high water, and how high differences are on the maps.
How this affects zergs Roamers Scouts etc
donate salt to salty asuras, I got lots of them to feed