Koh Tonsay is a 20 minute boat trip from Kep beach for around US$8-10 return if part of a tour; alternatively, get to the boat pier before 09:00 and share a $25 boat ride (to go and come back) with others (maximum 6 people). Many people just book a day trip to the island and don't stay overnight. Beware of buying a ticket in advance for specific time through various agencies. Apparently, they do not have any scheduled services. The staff will try to arrange a boat for you, but in a way, that they are doing you a favour. Any complaints about the scheduled time will make them angry, and you'll be told to complain to your agency.