Staple a background of blue construction paper with touches of glitter near the top to simulate the reflection of sunlight off the ocean surface.
Cut colorful images of fish, seaweed and bubbles out of construction paper following the patterns on the templates or coloring pages. If you'd like, include other undersea items like coral formations or treasure chests. Staple these around the edges of the board.
Choose the most interesting or informative photographs, images, charts and text blocks that reinforce what you are teaching in the current section. Arrange them on the bulletin board, doing your best to create a visual balance between text and images. For younger classes, you could substitute pictures or crafts that the children created.
Cut letters for a title out of construction paper and staple them up at the top of the bulletin board. The title could relate to the current unit, like "Formation of Coral Reefs," or you could simply use "Under the Sea."