I remember Morrowind and Oblivion atlest having a few but I've yet to find any in Skyrim. There's almost no reason to explore underwater is there?

Ilinalta's Deep and Pilgrims Trench are technically underwater, though both of these places can be found while your head is above water. I think it would've been neat to have a few underwater-only places to explore though.

yeah, considering argonians main ability is water breathing. you'd think we'd get more reason to use it lol

Where is Pilgrim's Trench?

Northwest of Winterhold, nearly adjacent to the tomb of Ysgramor.

e: the reason why I mentioned it is because it is impossible to explore the wreckage without a waterbreathing enchantment or playing as an argonian.