Earlier quoted context omitted.
Slightly off topic, but you should shower to avoid temperature difference too, specially in hot days. I don't know about the lifeguard situation in USA, in Italy it's pretty terrible; it should be absolutely required to have a shower before jumping in the water.
Could you explain the thing about the temperature difference? What's that about? Also, where do you have lifeguards? Indoors, outdoors? I don't remember seeing any at all here in Sweden.
The problem here is that a lot of people doesn't know what's going on with their hearth and certainly a lifeguard doesn't know about.
Also, adding to that, https://en.wikipedia.org/wiki/Cold_shock_response
I don't know about Sweden situation, but it seems really weird that no lifeguard was there. In Italy every public or private place with a pool needs a number of lifeguards according to the number of people of the pool / the pool's dimension. It's really easy to have some heart attack or hypoxia going unnoticed, example: X person jump in, simply doesn't get up. Also, time is key, if an heart attack occurs a close defribillator with someone able to use that / a lifeguard able to transport that person out of the pool without damages (it's not that easy as it sounds) can be really important. Drowning situations are even more dangerous, a drowning person can have a force in their arms able to do real damage and everything needs to be deal in the safest way.