I purchased The Temple of the Sea God as a battle map for the pirate god’s temple from Freeport. There’s not a lot of work with to create a water temple – pools are a given, so that leaves other “sea god”-like paraphernalia: tridents. There are only five tiles in this set and every one of them has a trident in it. The altar features two tridents and two mermaids. As a generic water temple, this is an excellent supplementary set. But if your sea god doesn’t use tridents then you’re out of luck.
You can order Temple of the Sea God from DriveThruRPG.