Sure swimming is good for you. But if you want serious changes to your dive propulsion abilities--20 second durations, 2 muinute durations, or 1 hour durations, then structured cycling training is what you should be after !
That's why I do spinning 3-4 times a week (with many intervals, sprints, climbs, etc), and then do long rides outside on the weekend. Swimming was more to keep my upper body in shape more than I am now, not so much for diving.