I think I just came across the definitive answer to my question : https://help.openstreetmap.org/questions/65910/should-islandsislets-be-the-inner-border-of-naturalbay-areas?
Frankly, I would prefer to convert the multipolygon into a node because I think it would be better in all but I am too beginner to take this initiative without your advice: Yes or No?
Better still, would someone with experience want to do it properly?