1...small children don't realize they are getting tired. Anyone who has watched a 3 year old knows they can run in circles for hours. However when they are swimming and they get tired, they just sink. That's why frequent rest periods are necessary.
2...that just because they learn how to swim one year does not mean they will remember how to swim the next year. I speak from experience.
My friend used to have a pool. Her daughter, Alexis, started swimming on her own (supervised) when she was 3. Every year she would have to learn to swim all over again. She would start out playing in the water on the pool stairs and it would take her a couple of months to learn to swim again.
She used to love to play dolphin with me. I would put her on my back (piggyback style). Then I would dive to the bottom of the deep end of the pool with her on my back. At first I would come right back up but I would gradually increase the time we spent at the bottom of the pool. She knew that all she had to do was to let go of my neck and she would float back to the top of the water and I would be right there to help her if she needed it. We had a ball like that.