Provide all the materials preschoolers would need to create a tropical island in the sensory table: sand, blue-tinted water, paper umbrellas, cut-up wash cloths for beach towels, and little people to play on the beach. Have the kids mound the sand up in the center of the sensory table and then pour the blue-tinted water all around the sand. This teaches children that an island is a body of land surrounded by water on all sides. They can use the little people to narrate stories on the island's beach.
Especially if your preschool is located far away from a real beach, show the preschoolers pictures of sand castles because they may have never seen them before. Add enough water to the sand in the sensory table to make it stick together when you make a ball, but don't add so much that there's any standing water. Provide kids with various buckets and cups so they can make different shapes with the sand, and demonstrate how to fill the buckets and tip them upside down so the sand holds the buckets' shapes.
Fill the sensory table with water and then drop in items that might be seen under the sea. Seashells, natural sponges, interesting rocks, starfish and plastic fish are good props for this activity. Since water may spill out of the sensory table, lay a plastic tablecloth or tarp under the table before filling it with water. Enrich this activity by talking about the different kinds of creatures that live underwater.
Fill one end of the sensory table with sand to make a beach, and fill the rest of the table with water. Add a piece of wood that juts out from the sand into the water. This is the dock. Provide plastic boats for the children to play with, and explain that boats have to use docks when they approach beaches. You could use this as an add-on activity if you have previously made boats as a classroom craft. Children can make boats out of plastic water bottles, folded paper, egg carton cups or polystyrene packaging.