My son is a big fan of any and every underwater animal, movie, show, book, song, etc etc. When planning for his 3rd birthday, it was easy to pick a theme that was totally him. I think it's important to incorporate your theme throughout the party to really delight the children and give them the full experience. Here's my attempt at doing just that...
When my the birthday boy came downstairs in the morning to see his party set up and exclaimed, "I'm under the sea!"