From the yt comments:

One of the crew members' memoir (Эдуард Овечкин "Акулы из стали") mentions that the pool was filled with freshwater. The crew rarely used the pool themselves, because they could find better use for that much water. The author had an interesting story about this swimming pool. One day, a high-ranking officer came with inspection. He was very rude and the crew didn't like him in return, especially since he sat in the captain's chair (only the captain was supposed to sit there). Then this officer wanted to take a swim and he ordered the crew to prepare a pool. As the author was drawing water, he and other crew members decided to urinate in the pool. And then watched as this officer was swimming there, barely containing the laughter. When they finally told the captain about this in control room some time later, the submarine was sailing without control for several minutes, because everyone was laughing on the floor.

It's like the old public swimming pool joke. "Check the pH!" "Hmm, mostly p, not much H."