What i did for my beach wedding seating chart. Buy a bag of starfish (dried out of course). Get a lot of ribbon (the color of your wedding) and tie the name tags onto the starfish. I put the names on the front and their table numbers on the back. Then you get a box, put sand in it and stand the starfish up in the sand.
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.