In reply to mrgleb:
Be very careful with your shoulders. I was a club level (300-400 lengths p/w) swimmer and I had rotator cuff problems before getting into climbing, which have since cleared up since stopping swimming. I still go once or twice a month but find that my shoulders really flare up quick, especially if I have been climbing a lot recently.
Personally if it's a keep fit type thing I would go for steep hillwalking like others have said, but If it's swimming vs cycling I guess swimming would be the 'best' option.
Just make sure your hand isn't entering the water too close to your head and then extending underwater before you pull back (on front crawl), this is the main way to wreck your shoulders. If you concentrate as reaching as far forward as possible with your elbow 'high' you will probably end up with good technique and not-destroyed shoulders. Also try and do other strokes! Backstroke is amazing for me, stops me getting the climbers hunchback and is a relief from front crawl. good luck