What are the dangers of wearing wet underwear?

No, it won’t damage your testicles, and it won’t hurt unless extremely cold.

I wore river rafting shorts with a mesh liner during daylight for three days in Hawaii, jumping into the water (both salt and fresh) every couple hours. No damage at all.

Wet cotton underwear does not dry very efficiently when worn. If it is damp at 10 a.m., it will still be damp at dinner time.

One time events are no biggie….just dry them as bet you can. Wearing wet undies will be a nice breeding ground for bacteria and if you do it regularly enough I would be on the watch for rash and fungal infections. IMO no underwear is better than wet underwear.

You might get a yeast infection.

Diaper rash.

Jock itch
Uncomfortable but testicles will survive the evolution.

I always thought that you would catch cold down there, I guess that is an old wives tale.

You can’t wear anything over that, like pants and skirts (pr blouses, if yo’re a girl), because it will get wet too and people will notice you wet ass (and other parts). Also, you can’t sit anywhere either, you’ll leave a mark, either on the place you sit in or on your ass.
Also, it’s uncomfortable.

Nothing. You swim, don’t you?

It only takes about 30 minutes to dry. Not enough time for diaper rash or bacteria. Plus it isn’t just the wet that causes diaper rash. It’s the acid in the urine and feces sitting up against the skin for a long time.