"Professional" means a lot of things. It can refer to specialized training required to do a job, it can refer to being paid to engage in an activity most people do on a recreational basis, and it can refer to the conscientious manner in which someone performs a task in accordance with best practices.
Getting paid well isn't a necessary part of any of those meanings.
As a general rule, it's very difficult to make a living doing something that millions of people love to do for free or at great expense to themselves. This rule applies to writers, musicians, athletes of every sort, skiers, surfers, anybody who races anything, crossword puzzle enthusiasts, actors, dancers, and beer tasters. For all these groups and many others, the median net income is zero.
I think most divemasters and dive instructors (at least the ones who don't take the plunge to buy a shop or a charter boat) quickly acquire a pretty realistic idea of their earning potential, but that realistic view does not dissuade them from approaching their jobs with a professional level of commitment. Many aim no higher than to dive locally for free or to earn enough to cover some of the cost of gear, continuing education, and dive travel. Those modest financial aspirations do not make them less professional or less than professional.
That's why I don't think it's disingenuous or dishonest to refer to divemasters or dive instructors as professionals or to promote professional standards and levels of care among them.