If you like swimming, but do not have a pool in your gym or it is too small and crowded, try a 50 meters, 9 lanes pool in Luzhniki sports cluster. I have been there both during the week (in a day time) and on weekend and there were never more than 2-5 people on the lane. The pool is airy, spacious, with lots of light coming through big windows, clean and does not smell with chlorine.
Its infrastructure though is slightly confusing. The changing room is on the ground floor, and to get to the shower and the pool you need to walk two floors up the stairs. It feels strange to wear a swimming suit walking the corridors very similar to an office building. I had a feeling that if I open the wrong door I will see people working at their desks.
Actually, Luzhniki pool was first opened in the 1950ss at the same site. It was an all year round open-air pool - you could swim and see snow melting on the water surface. It hosted Olympic competitions in the 1980. About five years ago they took it down and built it anew adding "just a few things":
A smaller pool of 25 meter and three lanes (quite packed)
Restaurant by Arkady Novikov
Water park, a smaller pool and saunas have separate tickets, so you should say you are going to a 50 meters pool when you will buying a ticket. It costs 650 rubles. There is no need for health certificate (it is in many other pools).
Luzhniki is a big sports cluster on the opposite side of the Moskva-river from Sparrow Hills (you can see its biggest stadium from the observation platform). People come her to run or cycle along the river, to swim, play football (there are fields to rent) or tennis. You can reach Luzhniki by metro - get out at Vorobyevy Gory station (line 1), or by car. Parking costs 100 rubles per hour during the week and 150 on weekend and days of big competitions.
We have already written about the beautiful outdoor pool in Luzhniki. It works only during the warm season and should open in May.