If you want to see an island of the Myeik Archipelago without taking an expensive tour, catch the ferry from Sake Nge Jetty to Kadan Island, an hour from Myeik. Don't expect to find any beaches: Kadan is mostly forest and mangrove swamps (good for birdwatching). The ferry docks in Kyunsu, the 'capital' of Kadan, where there are a handful of simple restaurants. Motorcycle taxi drivers meet the boat and will take you to Nawarat Waterfall for K10,000 return.
Note that foreigners are not allowed to stay overnight and that you may be required to show your passport on arrival. There are two ferries daily to Kadan (K1500, one hour, 8am and noon). A sole ferry returns to Myeik at 3pm each day.