Is your primary motivation for doing this because you like to help people in need, or is it simply a way you can afford to cruise
Like pretty much most other boat based money-making endeavors, I suspect if a person outfitted a boat as a dental office and appeared in harbors as a discount dentist the authorities might be somewhat reactionary.
However, if you now started laying the groundwork to be a travelling clinic offering dental care to the local communities that do not have any dentists there is a possibility that you would get the town welcome and official blessing in every little port you put in to. Keep in m ind many of your clients are dirt poor, but not many are lazy so your boat can benefit from a lot of cleaning/maintenance attention, you can have a steady supply of fresh food
, and in general feel good about yourself for doing worthwhile deeds.
I know a dentist who does humanitarian work in the Phillippines (IIRC, it's somewhere) and I'll ask him if he knows of any organizations you might be able to contact. I'd take the approach of finding a US based organization that can help or sponsor
you in whatever cruising area you are thinking of, and then through them approach the local governmental officials.