number of troops underwater with water mages

[General Tacticus]

Water Cult costs 50 design points, and require Magic +1.

It allows all sacred troops to enter Water within your own dominion (they will need a sacred commander or one that can go underwater, of course). If they are underwater in a province and your dominion there goes down to 0 or less, they drown instantly (so you had better take a strong dominion).
IIRC, missile troops lose their missile attack while underwater, and flyer may not fly underwater.