I agree that you really have to check out Oriental Trading Company!!
They always have a TON of beachy or Luau items. Along with the usual favors and projects, I recently discovered that they carry a huge selection of beads and supplies! There are some cute charms and beads that would work GREAT with your theme - glass bead sets like beach items like swim trunks and flips; another set that has pineapples and palm trees; various fish, dolphins, starfish or undersea creatures - lots of choices!
They're online at orientaltrading.com; click the craft supplies and hobbies tab at the top, and there will be a "beading" link under it. I'm trying to put a link in here, but for some reason it's not copying properly - but they're very easy to find!
You could get a roll or two of stretchy beading cord, lots of inexpensive plain beads in watery blues & greens or bright happy summery colors and then a few specialty beads per kid. Necklaces, bracelets, or a clip-on key chain/zipper pull might be a fun activity - with a built-in favor to take home!
Also, definitely try your nearest Dollar Tree soon; they still have pool Noodles - for a DOLLAR! That's a pretty high-impact favor for a great price! Can't beat that.
Good luck - hope your party is great fun!