There are a few ways to get around on the islands, generally, if you are going from mainland, you will take a high speed ferry, or speedboat. Avoid touts offering private speed boats as they will try to rip you off. My favorite are the high speed ferrys.. super cheap and relaxing. Once you are on the islands, you can hop around from island to island via Longtails which are pretty much available anywhere on any beach. Most of them will approach you. They are also pretty cheap.