Among the articles I've read discussing the regions of South America where coca was grown when the Spanish started exploring the New World, I haven't found any that mention it being grown on islands off the coast. I know that some drug plants are grown on islands as well as the mainland (like
Anadenanthera species), but I don't know if this is the case with coca.
The reason I'm curious is because of a particular account that's often cited as the first written mention of coca chewing (it's also sometimes mentioned as an early account of tobacco... but of course it's hard to be sure since the plant itself isn't described). It comes from Amerigo Vespucci (the guy that America is named after), and it describes a group of Indians that chewed green leaves constantly, to which they applied lime by the means of a stick moistened in the mouth. But when I went back and read the actual letters written by Vespucci, he mentions that this observation occurred on an island off the coast of South America, not on the mainland. That sent up red flags, since coca isn't generally cultivated at sea level (it is occasionally, but mountainous regions are much more common habitats). Between that and the fact that it occurred on an island, and I haven't encountered any other accounts of traditional use on islands, I suspect the green leaf was in fact not coca.