Just finished this galleon shipwreck:
I ordered an incomplete, used ghost pirate ship — set 4806 — on Ebay, a while ago ( $35 incl. shipping! ), and went to town with a dremel, jewelry saw, and acrylic paint. Took about a week, in my spare time.
The acrylic paint held surprisingly well to the unprimed hull. I started with a black wash, which I applied with a couple brushes ( one 1/2 inch, and the other one, small, to emphasize plank lines and cracks ), and then switched to an olive brown wash for the algae, using a torn sponge.
I finished by cementing PUA yarn ( which I dipped into the algae wash, to knock down the shininess ), and then a bit of model-making clump foliage, here and there.
Note: I was tempted, initially, to use a laser level to make a diagonal cut across the hull, so the ship would be more realistically tilted and sunken into the sand, but thought better of it, as that would make standing figures on deck impossible. This is meant as a playset for my nephew, after all.
