My son was about 21-25 months last summer, so about the same age as your daughter now, and he used the puddle jumper (the "are" pic in previous poster's list) then. We used floaties (inflatable sit down thingies shown in the other two pics in the list) the year before, when he was about 11-15 months. You still have to watch her with it on, obviously, but my son got pretty used to it and would try running/doggy paddling by himself when he floated. He loved it.

It all depends on how comfortable your daughter is with water. My guess is if she sees her small cousins/peers swimming without help, she's going to want to jump in without any assistance as well. She will find a sit and kick floatation device boring, so I'd go for the puddle jumper/life vest. She's old enough to want to move independently anyway.

We bought our puddle jumper at costco ($15 or so) and the life vest as well ($20, I think).

If you want to go really cheap, get a long and skinny pool noodle and tie it around her underarms. You have to definitely watch her because the noodle may untie, but if it's long enough the friction tends to keep it tied. It's smaller than the vest and puddle jumpers so it doesn't get in the way as much. BUT some parents will probably yell at me for even suggesting it since it seems so dangerous. My two kids did fine with the noodle method and I actually learned it from another parent at the pool. His daughter is now on a competitive swim team in highschool while his son was swimming the deep end at age 5. He stayed by their sides the entire time the noodle was on, and so do I! But we still used the puddle jumpers and life vests and noodles because my kids like to switch it up depending on who else is at the pool. You know. They see a friend with a noodle, they want to use their noodles. They see a friend in a vest and my two kids fight over who gets to wear the vest (we have one vest and one puddle jumper).

Hope that helps!