All you need is some duct tape and a rock. Be thoughtful with your choice of tapes and consider the color of the bottom of the pool. You want to pick something that you'll be able to see. (This presumes you have duct tape hanging around that house. If you don't, I weep for you. Now go get some!)
First we need to make a loop using the tape. Rip off a piece about 12 inches long. Fold one side lengthwise to the center and press well.
Fold the other side up to create a non-stick strip. Use a 1-2 inch piece of tape to join the ends together, forming a loop.
Now, put the loop on top of the rock and us coordinating tape to attach it. Don't skimp or you'll end up with a loop and no rock when you retrieve your toy from the water.
And there you go! Hours of diving fun! The rock will drop to the bottom and you need to dive in and grab the loop to retrieve it. Just be careful, of course, not to throw this AT anyone.
Here's one that my daughter made actually underwater at the Belvidere Pool.
Another super simple idea is to take an old plastic Easter egg, put a few small rocks into it and decorate it with duct tape. This is also really fun to dive for.