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.
Easiest option is the Tourist Visa Multiple Entry. Valid for 6 months and you receive 60 days on every entry. And an entry can be extended with 30 days at an immigration office (1,900 THB fee).
So in those 6 months you will need to leave the country at least one time and do a border run or make a city trip to a neighbouring country.
Anonieme deelnemer if the applicant has a valid reason to cancel, like a serious illness, then I would refund the fee minus the administrative cost you had.
But I would also warn him/her that Thai authorities will be notified and his/her DTV might be revoked. Not sure if you can do this, but it would deter people abusing your services.