A water hulk is a slightly stronger counterpart to the umber hulk in SLASH'EM. The only differences between them are that the water hulk has a higher level (11 instead of 9), can swim, and is generated only in Gehennom.
Encyclopedia entry Edit
Umber hulks are powerful subterranean predators whose
iron-like claws allow them to burrow through solid stone in
search of prey. They are tremendously strong; muscles bulge
beneath their thick, scaly hides and their powerful arms and
legs all end in great claws.