I like this idea, but would try to extend the water theme. Put plastic fish inside with a gift card to Bass Pro Shops! A gift card to the pool store or local water park would be fun too. I'm all about making the gift card connect.
Seashell Seating Cards 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.