I fly with them 4-6 times a year to BKK in BC. If your return ticket is more then 60 days, or you don’t have an RT ticket, chances are they ask for your visa or an onward ticket as they don’t want to be accountable if you would be denied entry in Thailand.
Do you have a property or a long term lease ? Show them a rental contract and a declaration of the condominium manager that you are/will be living on that address.
Either you’re doing the Divemaster course and pay for this course in full. You can do this with a tourist visa (single entry or multiple entry) or if you return within 60 days you can make use of the visa exempt.
Or you are doing your Divemaster course as an intern where part of your course is “paid” by working for the dive club. In that case you’re not a tourist but an employer…
In the past, most western Divemasters were working illegally and stayed in the country on tourist visa with monthly ‘visa runs”. But I wouldn’t take that risk anymore.