If by secluded, you mean no crowds, that's going to be hard. If it's an accessible swimming hole in Texas,it's going to be popular, but if you can go during the week and avoid weekends, a lot of these have small crowds. I wouldn't say there are any secret swimming holes you can get to. They wouldn't stay secret long. But we'll try to get close.
I'll list Inks Lake State Park, even though the park sometimes hits capacity and closes, mostly on weekends, because it's a big lake with lots of space, and a lot of people are there to fish. The lake has areas of cliffs you can jump from.
Devil's River State Natural Area - You want remote, this is it. You have to hike to the river, but you should always be able to find some space to yourself somewhere on the river. Nice river. Low water sometimes, but that's really a paddler problem. Dolan Falls is a prime spot, but worth it, even if there are other people there.
San Felipe Springs, Del Rio. No doubt will have a lot of people on weekends, but likely not so bad weekdays.
Kerr County has several swimming spots on the Guadalupe River, but most are busy places. Two that usually aren't are Wagon Wheel Crossing, Small dam. Shallow water. Good picnic spot. You can see it's kind of remote.
and The Flats, half a mile from Wagon Wheel Crossing. Underwater rock slabs form "tubs" you can relax in.
The Quince - River Road, Camp Wood
Utopia City Park - Another that's probably not busy on weekdays.
There are many other great swimming holes, but they are all going to attract crowds all summer. They're easy to find by searching Google for TEXAS SWIMMING HOLES.
GREAT list above!
No matter where you go, don’t climb over a fence to get there. Trespassing on private property isn’t a good way to meet people.
Like I needed to see that. Why did I click on that? Thanks 5thGen, no sleep for me tonight. Are snakes bad this year everywhere or just in my neck of the woods? Our next door neighbor has killed two cottonmouths in the last week in his backyard.
I haven't seen more or fewer particularly, although I've seen some doing things I didn't expect, Like the small rattlesnake that my neighbor spotted leaving one of his purple martin houses full of eggs after climbing a 2-inch steel pole fifteen feet into the air. If there are more, it generally means it's a good year for rodents, voles and mice and such. But I haven't seen noticeably more hawks, and the two go together for the same reason. And I don't know about your area, but around here, there's so much hay from last year in storage that a lot of pastures are being left uncut, making them snake heaven.