There is one daily speed-boat (40 minutes) from Sihanoukville at about 10 am from Thma Sa Pier (Phae Thma Sa in Khmer language), which is a 20 minutes tuktuk-ride from downtown. Tell the boat people you are a volunteer for us and you will get the local price of 7 US$. Here in Khum Thma Sa it's a twenty minutes walk uphill to our place...