Depends if you are talkin gabout shore diving or boat diving.
Shore diving, also depends if going in off a beach or rocks. Lot easier off rocks in a small swell than off a beach.
For boat diving, as a boat owner, my main limitation is wind speed and direction. Any wind over 15 knots from south in Sydney means I will cancel. 15 knots from west is okay as offshore. 15 from north not too bad and 15 from north-east can be bad, but depends. This is because these winds will make it so uncomfortable and also after a few hours of these winds blowing, dangerous.
As to period, I disagree with most of what has been posted earlier. Period is very important. You can have a small swell (say 1 metre) but with a long period have a very uncomfortable dive. Wave period is not really related to swell height. For example,
Wave Period Between waves Wave Speed Depth of surge
Seconds Metres Km/h Metres
8 100 45 16
10 156 56 25
12 225 67 36
14 306 79 49
15 351 84 56
As you can see, if you have a wave period of 8 seconds you will have surge (just) at 16 metres. But a long surge of 12 seconds will go to 36 metres and 15 seconds to 56 metres. This is not dependent on swell height.
Therefore, for boat diving, a combination of wind speed and direction, swell and wave height and wave period will determine if the dive is safe (and comfortable).
Even after almost 30 years of boat and shore diving, I still get it wrong sometimes based on the forecast, so it is not an exact science. However, I will say that in the past 10 years with all the electronic weather information available, we have very few days when we go out that we have to turn around and come home without diving.