I haven't heard of DMTs or instructor trainees getting ED visas. So if you're claiming that it is an educational course, you'd be hard pressed to get someone from MFA to agree with you. Anyway, DTV is a brand new visa, let's see if anyone gets approved for DMT or IDC as their main mode of acceptance.
I think the embassy might take a dim view of dive professional training though, since it implies the possibility of working in Thai dive centres. In the press release for the DTV, Muay Thai specifically was mentioned as sporting activities.
Scuba is not officially one of the "soft power" categories on the DTV, so it might not be accepted. Have you reached out to your local RTE for the requirements? You could potentially do a medical appointment then cancel it.
I can say for Singapore, as a Singapore citizen - the thai embassy in Singapore only issues visas for Singapore citizens and valid long term pass holders.