Yep, it's a pool that was filled with saltwater pulled in from Arctic ocean. There was also a sauna! The pool was even full while the ship was breaking ice... check it out:

https://youtu.be/tE4l5-flcnk

Cool. Was the ship rocking, and the pool water stationary? Or Vice versa?

Looks to me like the pool is stationary relative to the ship, so I would think that the pool water would slosh around. I'm not sure, though. Modern cruise ships have much fancier systems for that sort of thing.

this pool is there by design. On some mid-size and large USSR/Russian military ships which don't have a pool by design, you can find what your local building department would call "unpermitted remodeling" - as you can't really make a hole in a ship's deck floor (i mean theoretically you can, yet it would be a can of worms of totally different scale), the crew would build the pool up so there is only hardly a couple feet between the ceiling and the water and you get into it by climbing up the ladder.

Sort of like how my brother built a pool in his dorm room out of plywood and tarps.

On the second floor.

Maintenance was not amused.

Even the Typhoon-Class Sub's had one, and a Sauna ;)

Looks like a swimming pool, I assume crew and guests use it for exercise and recreation.