Preparation
Bring a source of underwater breathing. The easiest source would be a ceramic seashell necklace from a priestess of Ithrilis, found in Xaventry. The boss is located in A Den Woven from Living Kelp (30800) in Pirate's Cove.
from recall: n, 2e, 6s, se, s, 2e, 2s, 5e, and then the boss room is southwest.
Encounter
The boss does not cast or do any particularly scary dmg, just has a lot of hp. Whittle him down and flee out as needed to regen.
Loot
Scroll of excommunicate