Instead of numbers to assign guests their tables, use names of seashells. Each type of shell represents a different table. Arrange seating cards and shells on aqua fabric to give the impression of water. On each table, display a card with the shell's name, or fill a bowl with those shells as a centerpiece.
medium sized seashell dishes, fill them up with sand and anchor a colorful tea light candle within the sand. You can make this even more festive by placing two or three of them on top of a bamboo placemat and scattering a few rocks, starfish and flower petals around.