What do you think of this style of lifejacket/water wings? http://www.amazon.com/Stearns-Puddle-Jumper-Deluxe-Jacket/dp...
That's what we use with our two year old son, though he's also only in pools with one-on-one attention. I'd prefer to just take him to shallower pools, but his feet just don't touch in any of our local pools, and puddle jumper seemed like a good option for him to have a little independent exploration in the water.