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.
Seating Cards How-To:
Make simple rectangular tags on a computer (ours have coral-color words and aqua borders). For notched corners, clip with a 1/4-inch hole punch. Apply an adhesive dot to the shell's flattest part; it's removed easily, so guests can have their shells to take home.