Guys, guys guys...
A distinguished Diving Medicine Physician told me once, if u can't swin 200metres in 5 minutes (any type of stroke), then tak payah lah gi diving. This is so because if u get drifted apart from ur dive group, u will need the upper body strength to swim back to ur group, and if u're against the current, then GOOD LUCK...
Actually, that was not to scare u guys. I advise on swimming to build ur upper body strength sebab in diving, upper body strength is of utmost importance. Cardio workout may be find for total body conditioning, but swimming is the best for ur torso. Kalau lagi bagus, try doing >10pull-ups (palm facing forward - to build ur lattisimus dorsi muscles instead of ur biceps...) everyday.
Any further info on building upper body strength, please refer to Mens' Health mag, hehehe...