We read lots of books about the ocean from the nonfiction and ficition sections. Then the boys decorated a picture with stickers of underwater animals and glued on shells. Other ideas include:
Take a clear two-liter bottle and draw sea shapes (seaweed, waves, etc) on the outside of it with a permanent marker. Fill the bottle mostly full with water. Next add about a 1/4-cup of white vinegar and 1 Teaspoon of baking soda to the bottle. Drop a small handful of raisins into the bottle. And then watch. The raisins will collect bubbles all around them from the vinegar and baking soda chemical reaction this will cause them to float to the top. Once the reaction calms back down they will sink back down and then form more bubbles (causing them to rise again!) This will repeat for up to a half-hour. This is so fun! My kids just loved it.
Make an octopus windsock
I love this starfish craft: http://www.preschoolplaybook.com/2009/06/starfish-on-sand.html