For the U-505 diorama I used a styrofoam base, which I cut out where the aft portion of the hull was still submerged. I painted the styrofoam with acrylic "Plaid" paints from Michael's, then filled in the cavity around the sub with gesso, also from Michael's. The gesso comes in a variety of consistencies. I used the thick and thinned it with water for the filling part and out of the jar to finish off the top layer. The gel can be mixed with acrylic paints, so you can get some nice blending of colors. I use the gesso for all my water bases. Happy Modelling.
Phil