most of the big gyms throughout the city have a pool, but most are 25m max. For example most, but not all, Megatlons (a chain of expensive gyms) have pools. I tried for a while to find a nice 50m pool but couldnt find one that was convenient for me (I live fairly central), so i joined a gym in san telmo that not only had a 25m pool but also 3 floors of gym/aerobics/football/basketball etc for only 70pesos a month which is only slightly more than a membership of a pool. Of course it all varies greatly from place to place. If you need any more info just pm me.cheers.