I probably would not buy one of these for myself due to the price (this one was a gift), but definitely our favorite toy for kids and adults is the sea scooter. We have the Dolphin kind like this...
http://www.scootermotion.com/seascooter ... -zs06.html
I know it's expensive, I'm just saying, everyone loves it, it has held up for 5 years and the battery stayed good for 5 years, I just replaced the battery for this season.
Other than that... Goggles! Everyone wants to use them, if you can find ones that are easy to adjust, grab a handful of them.
We've also found that water guns are great fun in the pool, as others mentioned.
Enjoy!