This is a tough one. I looked at the link to DiveAdvisor (thanks for that BTW) and really liked the ease of navigating through it. You are right about the feedback, which is extremely limited. I suppose one of the problems is getting people to respond.
I would read the reviews on dive shops on TripAdvisor. They have always been very comprehensive when I have researched them. That said, between TripAdvisor and ScubaBoard, I don't know if I'd need/use another platform.
My problem with TA is that too many reviewers write of personal experiences of why they liked a shop, such as "the DM was so patient with my teenage daughter; I will definitely go back," or that, "this is the best" dive shop. Some give the info I'm looking for, such as boat type/size, does DM allow divers to go under on own or wait as a group, where are surface intervals done, does the shop do what they say they are going to do, etc.. Some reviewers do a good job, others not so much. Sometimes it's hard to get a response about specific dive shops, for whatever reason, on SB.
I would like to be able to enter month and water temperature, and a list of world-wide ranked dive sites would be generated.
Regarding a dive site, or area, this would be the type of info I would be interested in, but would like to search a specific site/location that I knew I was traveling to. Other helpful info regarding a site might be if it is a drift dive, if a wreck dive is penetrable, max. depth, possible life in the area, and formations like canyons and if there are swim throughs.
With sufficient data yes. But my concern would be the similarity to scuba earth and the difficulty to launch it with adequate motivation for users to generate content.
So my far fetched idea to generate sufficient data would be to get input from divers right after a dive. Dive shops would provide the opportunity to input data in some type of survey form. Knowing that's not possible, the user of the app would input data on their smartphone/tablet after completing dives. As motivation, provide some incentive for those who frequently provide input. For simplification (thinking of myself), have a limited amount of data that can be input.
Good luck!