Instead of just a single mobile hanging in a corner, you can decorate an entire ceiling with hand-crafted sea creatures. Give children pages to cut and color of different types of fish and let them embellish with glitter, sequins or other craft supplies. Upside-down paper plates with several strips of crepe paper streamers hanging off the edges make fabulous jelly fish. Tape crepe paper streamer "legs" to oval balloon octopi and long, thin balloon squids as well. Tape thread or fishing wire to the completed fish and hang them at varying heights from the ceiling. When air blows through the room, the fish will appear to be moving and swimming above.
Collect some assorted sea shells on your next trip to the beach, or collect some from a craft shop to make adorable shell sculptures. A small piece of heavy duty cardboard or scrap wood serves as a platform for the shell buddy, and you can let kids decorate their platforms any way they like. The first part of the shell buddy to lay down are the feet, which can be two small shells, or six sticks if you want to make your shell buddy a crab. Use a medium to large hinged shell for a funny-looking head with a big mouth. If you don't have a full hinged shell, glue two similar shells together. Glue the shell head on top of the feet. Let kids decorate the shell buddies with googly eyes on top of the head or yarn and feathers for hair.
Kids like treasure hunts, so give them the opportunity to go treasure hunting on the beach. Bring a piece of cardboard with you to the beach and some tack glue so they can make a collage of what they find. They may find shells, driftwood, dried sea weed or sea glass. Alternately, let them spread some glue around with a craft stick or make designs with the glue on the cardboard and throw handfuls of sand at it. Let inspiration guide them.
If you can't bring the kids to the ocean, bring the ocean to the kids. Collect some items from the shore, or buy things at craft shops, and put it out in bowls for the kids to use.
You've probably heard of a ship in a bottle, but instead of a ship, put a miniature ocean in it. Use a clear plastic bottle or jar with a tight-sealing cap or lid. Give the child things to fill it with: sand, small shells, pebbles, small toy plastic fish or glitter. Tint the water blue with food coloring before filling up the jars as it will have a nicer finished look than clear water. Before you close the lid, put a waterproof glue around the edge so it will remain shut. Place the ocean in a bottle somewhere the light will shine through it so it will look even more impressive.