While it's hard to regulate fishing when your country is made up of endless islands, it's quite easy to regulate dynamite. So no, I don't think the number of islands is an excuse.
In Fiji, for example, no fisherman could get dynamite if they wanted it. The only way to get that is if you run a (very heavily regulated) mine, or a quarry, and you'd have to account for every blast.
The fishermen don't use regular dynamite. They make their own with easily available agricultural and mining chemicals, which are impossible to ban on the island without other worse effects.