Following is a list of 5 ferry companies servicing the islands.
Cambodia Island Speedferry. Posted departures from Sihanoukville 9AM, 12PM, 3PM. Professional staff, large catamaran aircon boats enabling to travel at any conditions.
Romney Express Travel Professional staff. It is a large catamaran boat with TV, aircon and leather seats. Schedule is unknown.
GTVC speedboat. The fastest boats. Staff is professional, however, sometimes could be more helpful. Posted departures: 9AM, 12PM, 3PM.
Buva Sea. The company is linked to Virak Buntham bus company and sells a single ticket encompassing bus and ferry. This makes is quite convenient, however, before you book please read the public comments on the company on various forums. As of this writing the Tripadvisor and Google ratings are both 2.5. Posted departures 8:30AM, 2:30PM.
Speedferry Cambodia. The company has likely the largest network of resellers. Often they sell you a ticket but then place you on the boat of another company. The resellers are known to give out incorrect information about the departures and piers they dock. Posted departures: 8:30AM, 11AM, 2:30PM