In the future, you'll be flying right to Koh Rong International Airport. But not quite yet.
Koh Rong Ferry to Koh Rong leaves 8:00am with pick-up starting from 7:15am & at 2:00pm with pick-up starting at 1:00pm. The trip takes about 2 hours, pier to pier. Return Sihanoukville: 10:00am & 4:00pm from the pier at Koh Rong Village. Includes transfer back to Ochheateal Beach area. The boat tickets are currently $10 per person each way. You'll want to get your ticket in advance, as the boat does fill up.
Other than the Koh Rong Ferry, fast boats and slow boats go out to the island most days. On occasion, you'll have to wait a day or two to go out to the island, (or come back). This is mostly in the months between July and November, but can be anytime in the year, due to weather conditions, like high winds and waves.
All the bungalows on Koh Rong Island have their own preferred boat trips. Usually $10 to $25 for a round trip.
Also, private boats can take you out, around and back.