I don't own a swimming pool (I live in rainy Seattle; can't afford one; and live right next door to an elementary school!) It's a bit early to start thinking about swimming, but I like to be prepared ahead of time.
My nephews love swimming, so we end up going to hotels for special occasions to use the hotel pool, or borrowing friends' apartment pools when we can. (There's a city pool nearby, but it gets too crowded and crazy, so we skip it). Sometimes we are offered the privilege of visiting relatives with swimming pools.
When that happens, I like to bring fun and different swimming pool floats for the kids to enjoy. (I sure get tired from blowing them up every time though!) No boring round swim rings or inner tubes for us!
Some hotels even don't mind if you bring a float toy, so I bring some to share with the other children that will inevitably be staying at the hotel.
Here are some of the fun pool floats I'm considering this year, since someone naughtily jumped on and popped our Space Shuttle float we were using last summer!
Be sure to supervise your kiddos while they are using these. Toddlers especially should have Auntie nearby inside the pool ready to catch them as the toys can get slippery and the kid can slide right off. Also some of these toys cause the kids to fight (friendly at first) on top of the boys - a prime reason I no longer bring "noodles" to the pool, as it makes my nephews start bopping each other on the head and swordfighting!
Note that you can win massive aunt points for actually getting into the water and playing with the kids. Often times kids' parents are just understandably too plain tired to get in the pool. At hotels, I've noticed a lot of the moms gratefully sinking into a patio chair with a magazine while Dad takes the kids into the pool. Be sure if you're along with Mom and Dad that you help supervise the kids and are extra attentive - Mom and Dad might assume you'll do all the work, and lose themselves in their magazines and smartphones, exhausted and grateful for the break you're providing them.