If you want to see an island of the Myeik Archipelago without taking an expensive yacht tour, catch the ferry from Sake Nge Jetty to Kadan Island, 45 minutes away from Myeik. Don't expect to find any beaches: Kadan is mostly forest and mangrove swamps (good for birdwatching). But the locals are friendly and you'll get a taste of island life. The ferry docks in Kyunsu, the 'capital' of Kadan, where there are restaurants and shops. At the time of research it was possible to hire motorbikes (K6000 per day).
Note that foreigners are not allowed to stay overnight here, although that may change in the future. There are three ferries a day to Kadan (K1000, 45 minutes, 8am, 11am and 3pm). The last ferry back to Myeik returns between 3pm and 3.30pm.