My son loves this kind of thing. We started putting him in it last summer, so I guess he would have been just over 2.5 years. He loved the fact that he could splash around in the pool without one of us adults holding on to him. I believe we got it at Costco for not too much.
These are need for floating on and your little one can start practicing balancing. Just make sure to use it away from the edge and stay close by.
here a couple of really good ones. I bought mine from Toy R us $20
There are floaties that you can buy that are cute animals such as ducks, or just simple rings, that have a plastic bottom with holes in it for legs to go through. I used one when I was a child before I could swim. The child simply sits in the seat and it keeps them from tipping or sinking, as if it is an entire waist floaty.