Decorations can vary, but a universal color for under the sea themes is blue. There are many shades of blue available, and mixing them all can bring a more realistic color scheme to your prom. Cover the walls with shades of blue and turquoise to simulate water. Add in some clear twinkle lights. Include decorations that look like treasure chests, fish, bubbles and coral. Or you could use a more nautical approach. Purchase some backdrops that look like ships and add anchors and ropes to your themed decor. Table decorations can be anchors or small bowls filled with marbles and real goldfish.
Serve seafood. Appetizers can be cold shrimp cocktail and small crab cakes, and small bowls of Goldfish crackers can be set out on tables. Desserts can be cupcakes or cake decorated with sea themed decorations and frosting, or blue raspberry sherbet in clear glass dishes.
Send prom guests home with underwater favors. Have special frames made that show an ocean style picture that the prom couple's photo can be slid into. Include the date of the prom on the pre-printed frames. Let guests take home a small fish in a bowl as a favor, or get labels made that can be put on candy bar or lollipop wrappers depicting fish and the date of the prom. Give everyone a bath kit that includes soap, bubble bath and bath salts.