First, as DevonDiver suggested, I suspect you'll get more participation if you host this online somewhere. Second, might I make a few suggestions? (I'm going to assume you answered yes and go ahead and provide them...)
1) Your title implies you think divers (particularly rec divers) ARE beneficial to the underwater environment. It might be less biased to state it as "what impact do recreational divers have to the underwater environment" or something similar.
2) As g1138 indicated, there are literally millions of divers worldwide that have zero affiliation with PADI. Consider opening this up a bit and ask a more generic "have you taken qualifications in buoyancy outside of your initial training" or something similar. I'm not familiar with the other projects you list as being PADI affiliations or not but I suspect they're not strictly PADI things. Opening this up to all the other agencies and similar conservation projects by adding an "other-please list" category to question 1 might be good.
3) I'm curious what the air type has to do with conservation. Are you asking this just to help determine bottom times? If so, I'd think asking what a diver's average dive count and bottom time per dive is would be a more effective and realistic determination.
4) Consider opening your question 4 to "do you dive locally or travel more?" This opens it up to everyone other than just UK folks, unless you're looking for UK specific data. Also, add "other-please specify type and time" to your travel methods question. Some people use bikes, motorcycles, trains, etc.
5) Consider swapping question 7 with 8. I didn't know what citizen science was to answer question 7 until I read question 8.
6) Combine questions 9 and 10 as they are really the same question.