As a teacher, the hardest part of presenting a new learning experience to children is making it real. Some preschoolers may have no notion of what the ocean actually looks like or what lives in it, and showing pictures to this effect is fairly abstract. Set up a small tank with sand and water inside. Pile the sand on one side to create a beach with the water pooling deeper on the other side, creating an ocean. Show the preschoolers how to make waves with the water. Ask them how it affects the mini-beach.
A preschooler's day inevitably includes story time. Build your own ocean fish activity around the book "The Rainbow Fish" by Marcus Pfister. Read "The Rainbow Fish" at story time, then move to an art table, and ask your preschoolers to create their own rainbow fish. Show them pictures of brightly colored fish that live in the ocean, including the banded rainbow fish. Provide paper, crayons, scissors, glue, sequins and glitter. Incorporate the story's moral of sharing should any sequin or glitter fights arise.
Preschoolers are beginning to be able to take on developmentally appropriate responsibilities. In their classroom, they are expected to clean up after themselves, and at home, there may be similar expectations. Incorporate this readiness for responsibility by bringing in a hermit crab for a week, letting the children take turns caring for it, watching it move, and feeling its shell. Use this time to explore the environment in which the hermit crab lives, how it grows and finds new shells, and its diet.
There is a large variety of animals that live in the ocean. Explore a globe to let your preschoolers see the reaches of the ocean. Show them pictures of animals that live in warm parts of the ocean, like hermit crabs and tropical fish, as well as pictures of animals that live in cold parts, like penguins and whales. Using paper plates and art supplies, encourage your preschoolers to create the faces of animals they would like to pretend to be. Act out the animals together, and teach the preschoolers the different behavioral characteristics of their chosen animal.