We have tried many things, but I HIGHLY recommend the tube trainer by swimways. I think you can find it at Target and maybe Babies R Us. I am not sure what it is called-maybe a swim shirt?
It looks like a tank top with an inner tube built in. The tube sits around the waist/belly. it is inner tube material, so it is very sturdy. Since it slips on like a shirt or tank top, they cannot get out of it. It is VERY sturdy in the water. Unlike many things we tried, my daughters (3 and 20 months) do not tip over in this and they are able to move about the pool like little champions without us having to be right next to them every second. Highly recommended! Good luck!