sunken uboat

I'd try Lightweight Hydrocal.  Comes in very fine powder but after mixing it with water it has the consistency of milkshake.  mix in dye to for desired colour.  it'll expand a tiny bit but once it dries it has a matte finish.  

woodlandscenics.woodlandscenics.com/.../1

Now, I've never done what you're attempting, but after using it myself it appeared to be appropriate for the surface you're asking for.

My vote would probably be plaster, but that's what I have on hand. I imagine once you have the seawater modeled around it, the bit of haze you find at depth would obscure the silt such that no matter what you use, it'd look pretty good.