Helluva thing.
The SubSafe program came into being because of this tragedy.
In Enginnering training (nuc training) on the boat I was on a few years later, SUBSAFE was a big deal.
On a 616 class boat I was on, there was a panel that allowed isolating all of the aux seawater valves. It was located between the EOOW (Engineer officer of the Watch) and the Electrical Plant Operator who sat in front of the EOOW.
Another person mentioned the emergency blow capabilities. To prevent the air from freezing when it was dumped to the ballast to expel seawater (the rapid expansion apparently froze the moisture and stopped the expulsion of seawater), air dryers were added. But even more mods were done than that.
Another item mentioned here was the reactor trip. On the Thresher, once the rx tripped, you isolated main steam so that positive reactivity would not be added when cooling the steam generators as steam is drawn off. Back at that time, they were probably still scared of a SL-1 type of restart.
I am sure we were told the Thresher got within about 10 feet of the surface but with loss of blowing the ballast tanks due to freezing, no propulsion due to the rx trip and isolating main steam, no procedures for fast scram recovery, they were doomed to die.
We did fast scram recovery drills so often I still remember that it was easy and boring.
We were told in the yard that workers hung heavy rigging off an aux seawater pipe which stressed the pipe and that probably caused the failure. Hence you made some of the civilian workers go to sea to make sure the yard workers didn't do idiotic things like that.
Lots of things were changed to make a dangerous occupation a lot safer. It's a bad thing to kill over a 100 people AND leave a reactor on the bottom of the ocean with a lots of high level contamination. Adm. Rickover surely wanted to keep his reactors off the front pages of every newspaper in the world.