Vodyanoi
The vodyanoi (named for a Russian water spirit) are freshwater aquatic versions of the umber hulk. They lack the confusion ability of umber hulks, but can summon electric eels once per day. Saltwater vodyanoi twice the size of their freshwater brethren are rumored to exist.
In 3rd edition, the vodyanoi is a completely different monster. It has nothing to do with the umber hulk, being different looking and a fey-type. It is found in Frostburn.
Read more about this topic: Umber Hulk