I have two stories to share....
The first is about my uncle and his youngest son who was about 18mo at the time. My uncle owned a house that had a hot tub just outside the back doort with a pretty garden around it....no fence. My uncle had two children who were a little older...about 8 and 6 at the time. The 8 and 6 year old ran out the door and left it open. The 18mo toddled out the door and went after a ball that was floating in the hot tub and drowned. He never forgave himself.
Please fence off your pools and hot tubs....it can happen in the blink of an eye!
The second story is about my son. We were at a hotel and all of us were swimming....myself, my dh, our 5yo, our 3yo, and the 1yo. The older two kids had floaties on, and we were in the pool with them. They had out 100% undivided attention. We decided it was time to get out and dry off. We took the floaties off of them, and not two seconds later my 3yo just jumped in.
It took a second for it to register in my brain what had happened. I yelled to dh to grab him. At about that point the 3yo comes bobbing to the surface trying to swim....he was doing a pretty good job to. Dh grabed him and pulled him out of the pool. I gave him one of these
He scared the crap out of me. It really does only take a second.
One thing I have found that helps be be better concentrated on the kids when they are swimming is to be IN the pool with them. Its a lot harder to let your mind wander or to be distracted when you are in the water with them. Besides, it a great way to play with your kids and be involved with what they are doing.