I have dived around these islands several times a number of years ago on boats from Koh Chang. At the time I thought the diving was no better than what I had seen around neighbouring Koh Chang, but at the time the islands (as was Koh Chang) were very quiet unspoiled places so it made for a pleasant dive trip.
I went to Koh Mak for a non diving break last year to find the place is now full of resorts and tourists, so maybe not the tropical paradise you think it is.
I feel you may be just as well suited to one of the few resorts in the East side of Koh Chang. Much easier access, with the benefit of access to all the things available on the west side of the island. Salakphet is a personal favourite of mine, very quiet, nice and inexpensive accomodation and a wonderfull seafood restaurant on site. There was a small dive operation there also, but I know little about them, and I think they simply went to the same dive sites as all the shops operating from the west of Koh Chang, so maybe not as quiet as you are looking for.
With regard to the diving and your holiday in general, check climate conditions before making your booking, in the summer the area is prone to very heavy and prolonged rainfall, also rough seas. hence between may and september many dive shops (as other tourist related businesses) simply close down for the rainy season.
Hope you find what you are looking for.