We made a little island of acrylic "sea glass," surrounded it with a long string of plastic silver
beads Christo and Jeanne-Claude style, then marooned some shells (and cockscomb in bell cups) on it. The thing that really seems to hold this all together is that ring of beads.
An archipelago of islands like these with candles planted in the middle would make a spectacular accent if you are going for a beach or nautical feel. For a more natural look, try using sand surrounded by rope.