GREAT BARRIER REEF THEME IDEAS ?

I need some ideas for a low budget Great Barrier Reef themed Engagment party. The set color is Royal Blue with accents of bright yellow,red and orange.I need to find away to potray the vivid and magical color of the australian coast with out fishbowls or any seashells. I m trying to turn a campground pavilion into the carl reef with out heading overboard and tacky. PLEAS HELP.