Three creatures are commonly encountered in Vvardenfell's coastal and water environments. The mudcrab, and its larger cousins the king mudcrab and mudcrab titan, are large but unaggressive. Slaughterfish are small but very aggressive, and the larger varieties, the blind slaughterfish and the electric slaughterfish, are quite dangerous. But the most dangerous water creature is the large and powerful dreugh and the dreugh man o'war.
Spoken of in in-game books but not actually making an appearance, the Dreugh Man o'War is supposed to be the king of the waters surrounding Vvardenfell, and a formidable foe to even the hardiest of adventurers. This thing is massive and deadly, so makes sense it would swim in more open seas instead of the sheltered coves regular dreughs frequent.
I thought it might be fun to concept it up regardless of whether it will ever be included in the release, and I'm reasonably happy with how it turned out:
Higher res: http://postimg.org/image/h90eas5wv
I based its body-structure on Ravanna's already completed Dreugh model here, so they still look related. Maybe even a Bull/Alpha Male/Elder of the same species. I'm imagining the crest on its back is used to attract mates, and along with the top of its plating it sticks out of the water, giving it it's name.
Any feedback is welcome!

Damn, this is looking really
Damn, this is looking really hot. I'd love to see it done if it's possible, unfortunately there are probably more urgent models to be made... Still, this looks awesome.