I did last year on the way to Colorado last summer. Absolutely beautiful water and cold (62 degrees). I went at 6 am (7 am Texas time) to beat the scuba traffic. I only lasted two minutes in water before having to get out. Worked my way to staying in for 15 minutes before needing to get out and then would get back in.
Called them this year and they said no out of state visitors and they were booked up with scuba reservations through October. Could have probably done the 6 am visit again, but decided to go another way thru NM. Swam in Roaring Fork River in Colorado for the two weeks I was there, but waited til afternoon for the sun to warm up the water a little. Also swam in some of the gap lakes. Unfortunately, that area of Colorado just had devastating fires.
Isn't Balmorhea still closed? We used to stop every time when driving west.
I try to swim in natural water daily and living close to the San Marcos and Comal Rivers with constant temps makes that possible in normal times. When they opened state parks in the spring, we went to Lost Maples and did a creek walk along the Sabinal and found some nice spots to swim, but that's too far for a regular excursion. Under normal circumstances, we'd do a day trip and go to Wes Cooksey park to swim in the Nueces and to Fort Clark Springs in Bracketville for some good swimming spots.
I have done the slime bridge on Blanco and Kendall County parks for the Guadalupe. But the water is so warm that it's just not an enjoyable swim. Have been going to Landa Pool regularly, but that ends on Labor Day. The March shutdown was much better as there were no fences limiting access and I would just go at 6 am to get in.