Actually, I think that the ideal "underwater weapon" would be a spear (or something with a similar animation). So if you can team up with someone making a spear mod and doing the animations, I think you would be way ahead.
*GASP*
Now you've done it! You've committed a cardinal sin! You mentioned... SPEARS!!!
Don't you know? Bethesda removed them for a VERY good reason! They removed them because... because... well, I don't actually know. But I do know that they had a very good reason! So they should NEVER be placed in another Elder Scrolls game... EVER! ;-P
Ps. The same goes for crossbows and throwing weapons

Oh! That is because we signed the secret "You cannot fight underwater conspiracy secret agreement". I would tell you what the agreement is about, but it's a secret.
Well, I'm not afraid to spill the beans! I'll tell you exactly what happened. Between the times of Oblivion and Skyrim, the Thalmor forced everyone to sign the "Blue Pact." It specifically forbids the harming of sea life while they are underwater. If you are not aware, the Thalmor formed Tamriel's first society to protect endangered species. And because they are no different than any other form of government, they have allowed politics and extreme environmentalists to take over their original agenda. Personally, though, I think the Altmer just find seafood breath offensive.