I found there is doubled seafloor error in Nibon Bay, just beneath the island of the Quest: A Door in Niben Bay.
http://img526.imageshack.us/img526/199/doorh.jpg
http://img709.imageshack.us/img709/6766/seafloor.jpg

It's caused by over height LOD mesh in DATA\Meshes\landscape\LOD\
I believe it can be fixed because Quarn has fixed some other similar problems in Topal Bay and the Abecean Sea by UOP.
http://www.uesp.net/wiki/Tes4Mod:Unofficial_Oblivion_Patch/Statics_and_Clutter

I hope someone can fix the problem in Niben Bay as well.
Thanks!

I'm pretty sure that's happened every single install of SI I've ever had.

I have noticed this when I had Blood & Mud, Bravil Barrowfields, and the UL B&M Barrowfields patch. Never checked with plain vanilla though.