Interestingly, even if ScubaBoard membership aren't representative of the overall certified diver pool, they may be representative of a subset that is a desirable demographic to target with marketing and services.
Here's my thinking. If a lot of scuba cert.'s go to one shot 'try it out' people who won't stick with the hobby, then over time, a lot of people will be counted as much of the 'diving population' who are actually not diving. Unless you intend to make reclamation the thrust of your marketing, they may be 'dead wood.'
If the ScubaBoard membership are indeed avid divers, they may be representative of the avid diver population. If that's what you aim to market to, that's useful.
Does this matter if your main thrust is a marketing push to get new people to enter the scuba hobby?
It can be. If you just want to draw anybody & everybody, maybe not. But if you mainly want to draw in the sort of person who's likely to stick with the hobby over time, well, then, we must ask if the ScubaBoard membership is also to some extent representative of them in some useful ways. Or at least, could that which is compelling to SB members be used to season marketing to those potential divers you hope to recruit? Or make product/service offerings more appealing to them once they do walk in the dive shop door?
Richard.