Stu, was never trying to talk you out of your original plan, driving up, parking and wading with a fly rod.
Thousands of hours here doing that, since I could first drive and park at Pancranz crossing.
I know every bridge within 100 miles...
Certainly your highest probability of getting a big cichlid is Cibolo from the Boerne Nature Center (up the creek), and A-strain endemic bass.
The float adventure option may distract you from your focus of stealth and combing a few hundred yards of limestone creek, v. covering miles of it sporadically.
If you want to spend some time on maps and ask me about specific crossings, access, and fishing those crossings, please feel free.
This is Mueller falls up from Rebecca's Creek crossing (but you see by the bronze tint, this fish has smallmouth genetics).
A place we consistently catch cichlids the size of footballs are the lower Frio and Sabinal crossings near the confluence of the two rivers.
This is below where the rivers re-emerge from the aquifer, 15-20 mi or so south of US-90.
Just not a great place for endemic bass.
But there's sight-fishing to 5-lb largemouth there...
If you're interested in pay access (and even lodging), Shonto Ranch on Turtle Creek on the west side of Kerrville has purina-fed cichlids to about 3-1/2 lbs, they stock big rainbows in the winter, 5-lb catfish on fly rod, and big bass live there, too.