There is not really any reliable data for recreational diving as a whole other than perhaps what you may be able to squeeze out of various certification agecnies regarding certifications per year at vaious levels. But even if you get good new cert data, it is not clear how may of these ever dive again or how long the average diver actually persists in the sport. Persistence trends might be vaugely and broadly represented by the rates of advanced certifications, technical diving certifications, etc. However you cannot assume that if x number of divers have been certified in x number of years, that anything close to that number still dive.
Estimates of the number of divers who leave the sport vary widely and there is no real operational agreemewnt of what a diver really is. For example is a person a diver if he is certified but has not gotten wet in the last 2-3 years? The last 10?. Personally, I don't regard someone as a diver if they do not dive enough to keep their skills current and maintained at a level where they can comfortably do a recreational dive with no hand holding, which would eliminate a lot of recently certified divers. The point being is that there is no agreement on an operational definition of a diver and even if you find data, data from different sources may not be comparable due to different operationalization of the term "diver".
Also, in terms of the last 10 to 15 years I think recreational diving as an industry has been pretty stagnant so the effects you observe will at best be subtle and hard to tease out of what is already very poor and unreliable data. On the other hand there has I think been extensive growth in technical diving and I think the majority of equipment innovations or improvements have been focused in that area.
In line with this, and as a former college prof who taught reasearch methods and evaluation classes, I'd suggest that if you need a good grade on this project, you narrow the scope of your research and focus on a much smaller segment of diving.
Pick something smaller like technical diving, or even better cave diving, where you have a small number of certification agencies, a smaller and more dynamic population of divers to study that has a better chance of showing some clear trends. You will also benefit from much easier research due to a smaller number of people heading and/or administratively involved in the cave diving community - a community where for the most part all the movers and shakers know each other and are much more familiar with and aware of recent growth and trends.
In the US you also have the advantage of cave diving and training being centralized in north Florida and as such you can probably also access and use data from the state agencies in Florida that may have already collected data on use rates and the growth of cave diving due to it's impact on the management of publicly managed springs and recreation areas. If you are a cave diver in the US, the odds are good that you have been to cave country in Florida.
