I used to explain the mid day break to my kids this way:
At home we get up and are busy at school, work and afternoon activities during the day. In the evenings we relax and watch TV or play games. At Disney we're on a siesta schedule. We play from morning until after lunch, then siesta (tv, games, reading) while it's hottest. Then play some more well into the night.
The explanation helped them understand that we weren't expecting them to "nap" like babies, just shifting our normal schedule. In the end, they ended up falling asleep for a bit anyway.
I agree with others that think swimming is tiring. I really only find it fun and relaxing if I don't have anything scheduled. I hate trying to "relax" while needing to watch the clock.