Yarr, it's a party, mate.

 
One of my ex-roommates is having a party for her son with an 'under the sea' theme. I found this cute book at the library to give her more ideas. His party last year was actually pirates, but it's still applicable. 

 
How fun are these?! Fruit juice popsicles with cookie cutter-cut shapes of cantaloupe, honeydew, watermelon, or whatever other fruit you'd like to use. The book says these are "Titanic Treats...let's sink a few of these jiggling icebergs". 

 
"Shipmates' Snacks...we'll have a whale of a time!" 

Other ideas for the party include:
*Fishing game with a magnet at the end of the pole, and the kids can fish for prizes.
*Freeze sea creature-shaped gummis or fruit snacks in ice cube trays, then add the ice to blue punch.
*Or add the gummis to blue-colored jello.
*Serve goldfish crackers and other snacks in sand pails, or make a sand pail cake.
*Hang crepe paper streamers from the ceiling or a doorway, and clear balloons (bubbles) for the kids to walk through the sea. Add cardboard cut-out seaweed.

any other ideas I can pass along?