Really hoping it's just good bladder control...

If it makes you feel better: when you're swimming, you're sweating buckets. So most are probably just dehydrated.
- That was my first thought. When I was 15, I went to the doctor for vision loss and migraines. Always happened after surfing. He quickly pointed out that being in the water for hours and hours doesn't mean I'm not dehydrating and to come in for water breaks every couple of hours. Voila! Problems solved. Felt like an idiot.
- Also, a lot of people, myself included, make it a point to go to the bathroom before swimming.
  And I don't usually consume food or drinks until I am (almost) done with swimming. A four hour stretch of just recreational swimming and chilling is easy without food or drink.
- The secret is to drink lots of water before going in. Then you can pee without worry.

According to several sites, on average, there's about 8 gallons of urine in your average in-ground swimming pool or 2 gallons in your typical above-ground pool.
Luckily urine is 95% water and chlorine and filtration removes the rest.
- So it's safe to drink?
  
  eh... it wouldnt kill you.

"You can't pee in the pool!"
"But everyone does that...?"
"They don't do it from the springboard!"

The smell of “pool” is actually the smell of piss. Without piss chlorine has a much less potent smell.
https//wwwnpr.org/sections/health-shots/2017/03/01/517785902/just-how-much-pee-is-in-that-pool
- Correct me if I'm mistaken, but iirc it is the smell of the chlorine reacting with any pollutant, not just pee. Skin cells and skin oils will make it smell like that, too.