Oh, whoa. In the Making Water discussion, Leo (Biggles50) suggested using a very similar acrylic paste for water effects!
It turns out there are a huge number of acrylic pastes available for creating all sorts of textures. Marble dust is often a primary ingredient. All the ugly, zero skill paintings that hang in doctor’s and lawyer’s offices–acrylic paste and cheap paint.
This seems like an excellent opportunity to solve two problems with one product. Since I potentially need a lot of material for future diorama work, I will go with cheap acrylic paste marketed to artists and perform some experiments. Maybe I can sell the experiments to doctors and lawyers! That would be awesome.
On the subject of dioramas, I may have also solved my basing quandary! My brother has some left over cement board from a bathroom upgrade project. The stuff is rock hard, impermeable to water, and nearly indestructible. 