I trained briefly in a health club/gym pool (not full length, had a sauna etc). I would usually find that after swimming in it for any length of time, inhaling fully wouldn't feel like it was providing me with the expected amount of oxygen - hard to describe accurately the feeling but it definitely wasn't just breathlessness from exerting myself. Didn't really hurt or anything. When I swam in a proper fitness pool however, there were no feelings like that.