What about a sea drake from the infernal city novel, it introdused it to TES lore and TESV takes place well after the time line of the book. Its like a crocodile, but with paddles instead of legs.
Also the infernal city novel added the Sheartooth to TES lore.
A fish found in the Sump on Umbriel; "maw full of teeth", at least fifteen feet long, long whip-like tail, dual swimming fins on the underside, "teeth would shame a shark", tough hide, soft belly; usually non-aggressive. It appears that a rare alchemy ingredient called "Sheartooth Loin" is harvested by Skraw for the kitchens of Umbriel.
Just some ideas, i think it would be sick to fight underwater. Blades would be a must for fighting underwater because a thrust attack would be the only really effective tactic.