We stopped on the way back from Caracol. It seems like a great way to cool off after a hot day; however, it wasn't all that hot when we stopped. So I hesitated, trying to decide whether to actually get in or not. Do it! It's much more fun if you actually get in the water. There are pools formed by granite rocks, and the river is a little faster in some places, but not deep there. So it's just fun to slip into the stream of water and let it carry you down 30 feet or so. It's deeper in other places so we were actually swimming without touching the bottom. (Be careful not to kick the rocks under water and be careful getting in because part of the rocks can be slippery.) We didn't stop at the overlook, which I sort of regretted because apparently from up there the water/pools looks kind of like half an avocado, which we were told is where it gets the name. (Rio = River (Spanish) + On = avocado (Mayan) + pools (English).)
The walk from the parking lot to the river pools isn't very long at all. But it's a trail, not somewhere you could take a wheelchair. They have some changing rooms there. We were told that it gets extremely busy during Easter week and in the summer, especially on weekends. We only had one other small group there, so it was really nice.