Sheets of insulation used in house construction make good material for building landscapes. I never buy it, though. I scrounge it. I'll ask at building sites if they mind if I salvage scraps. Or salvage it from the scrap pile (a contractor friend of mine has a very colorful expression for doing this, but I can't print it here). But you should be able to find it at places that supply hardware and materials for building.
You can cut the stuff with a knife, or use a heated piece of wire to cut through it.
I also salvage the styrofoam packaging from things like stereo or other electronic equipment. Cutting it is best done with a heated knife or wire, too, because it tends to crumble otherwise as you cut.
You could also try florist's foam, which is foam with a finer matrix, usually colored green. Florists use it for dry arrangements; they stick the flower stems into the block. It cuts pretty easily, but it also tends to crumble or give off dust, since it can be a little crumbly. But you can stabilize it with white glue, or acrylic gel (an art product) once it's cut.
Since you can cut or shape it easily, you can build up ground contours easily. In your case, I imagine you'll have the level of the water's surface, and then slope up one more level, at least. So, you'll have one piece of foam, for the base level, and at least another piece to represent the rise of the land above sea level, carved to present the contours of your beach.
Hope that helps, and we'll see what some of the other modelers suggest, too.
Prost!
Brad
