A pool
The first game is a simple twist on the classic ring toss. You can either float the rings on the surface of the pool or hold them up along the edge of the pool, and allow the participants to toss the water balls into the rings. You can mix it up by having the participants in or out of the water, and changing the position of the rings. Also, if you have the rings in the water, adding in a current with wind or by having a leader moving in the water can add an extra challenge as well.
For the second game, set up the empty bottles on the edge of the pool and pass out squirt guns to each participant. Have each person stand in front of a bottle (in the pool) and try to knock it over with their squirt gun to see who can knock their down first. You may want to add a little water (or other weight) in the bottom of the bottles depending on the wind and how difficult you want to make it. Then you can set the bottles back up and see who can knock down the most before their opponents, or assign teams and add a little food coloring to the water in the bottles, or any other variation you can think of!
"Lucky Duck" is another classic carnival game that works well pool-side. For the traditional game, distinctively mark the bottoms of one or two ducks and float all of them in the water, making sure that they don't tip over and reveal the markings. Let each participant choose a duck, one at a time. A prize is given if one of the marked ducks is picked. To make it more exciting, have one participant begin turning over ducks until they pick one of the marked ones. Then it's the next participant's turn to keep going until they find a marked duck. The game keeps going until only marked ducks are left. You can keep track of who got the most ducks, or just give everyone a prize (remember, I'm working with toddlers!). You can also assign 2 big teams and see who can collect the most ducks. You may want to consider marking a few more ducks for this game, depending on how many participants and ducks you have. You may also need to mix up the ducks or have the participant spin around a few times so they don't remember which ducks are marked from the previous turns.
I considered a few other games when making my final decisions, like a version of pin the tail on the donkey that was waterproof, and having the tails on the bottom of the pool and using velcro to attach them to the board. Also, a floating version of bowling, where the pins are on a pool float. You may need to stick the pins down with something or create little grooves for them to fit in so they don't fall over too easily on the water. Look to the internet and your childhood for other carnival games you could adapt, or use research as an excuse to go to your local fair!