There are meals to be bought on the islands however we would absolutely suggest bringing what you can to make your time in paradise a little easier and cheaper. First off, water is not available for free so bring a big jug to stay hydrated. Bring snacks you can eat to avoid spending extra money, such as granola bars, fruits, sandwich fixings, etc.
Alcohol, along with most things on any island, is a bit pricier so of course it doesn’t hurt to bring a bottle of rum. If you are planning on a short trip out to San Blas, most hostels will hold your backpack for you in a safe place until you return so you can take a smaller pack. This makes it much easier to bring the extra things you need and not have to lug around all that extra baggage in and out of the boats.