When my son was a bit younger than your daughter, I got him a Swimways Spring Float, which I LOVED because of the support. It doesn't look fancy or supportive, but it really is. My son was able to sit in it before he was even crawling. You can find them at Target, or keep your eyes open at garage sales (that's where we found mine).
I also recommend staying away from those with tubes attached to swimsuits or swim tops. http://www.swimways.com/swim-sweater-p-20.aspx They may be great for older kids (my son is 16 months), but we tried one with my son, and not only was he able to slip through the tube (making it pointless), but he would roll so that he face was in the water as well (it didn't keep his face above water the way I had hoped). I would love to hear what people have to say about those swimsuits with the floaties in the top, because that's what we're considering next, but I would hate to make the same mistake we made with the Swim Sweaters.