Under The Deep Blue Sea
Neil MacQueen posted
If you have walls that can't be painted, then hanging a blue tarp works too. Paint a reef and fish on it with acrylic paints.
Fish nets, plastic fish, seagulls and shells can be found at hobby/garden stores and in party theme catalogs (such as Oriental Trading Co).
JCarey posted
Another suggestion for under the sea: Blue Plastic Tablecloth material is cheap.
You can hot glue gun "sea creatures" to it made out of vinyl or fabric.
You can also use large rolls of cellophane for under the sea effects. Green twist paper makes great seaweed when it is irregularly untwisted in small sections. Looks just like kelp. It's also lightweight enough to hang from the ceiling for a real "under" experience.
