They’re not exactly swimming pools, as in you wouldn’t really swim, it’s just a pool where you can dip to cool off after sweating it out in the banya.

The Typhoon class has a gym, spa and recreation area just like any nice hotel. If you’re trapped under the Arctic ice for 6+ months at a time, there has to be a banya. If not, the sailors would probably have rigged up a substitute using the nuclear reactor’s cooling system. Actually, I bet the banya is heated by the nuclear reactor..

This is the main recreation area in the banya. Is good for health, Comrade!

That the pool was there to help alleviate the stress of the crew. Being confined for 6 months is not easy and can bring on stress induced illness, which will degrade performance.

Also keep in mind that as Prasnath pointed out, these subs were designed during the Soviet era. Almost all navies in the world had only voluntary crews on subs. People CHOSE to serve, and were usually tested for compatibility before they were assigned.

The Soviet Navy, as all their services, had conscripts that served only 2 years. So their training was very short and they served only about 18 months after training. They did not have time or personnel to wait for volunteers or to test the sailors. They had significant problems with sailors who could not handle being confined for so long, so the ‘banya’ was added as a recreational area, with a small pool, to help the sailors relax and deal with the stress of the job and the confinement.

I have no idea if it worked and if sailors on Typhoon class subs did perform better, or had less stress related problems. It would be interesting to know.