They had many departure times which was convenient...

They had many departure times which was convenient, but not so well organised.

The 30min isn't quite accurate as they have many stops around Koh Rong and Sanloem. Our trip to the island was very full, with people standing and sitting along the aisles as many people entered the boat but not many left. Our return trip was fast and much more pleasant, with a bigger boat. Expect the journey to take up to 2h, especially if you are planning a connection in sihanoukville. Also, the boat ride back stops at another jetty further north, not at serendipity beach where you depart.