In the future, you'll be flying right to Koh Rong International Airport. But not quite yet.
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 $15 for a round trip.
Also, private boats can take you out, and the Sun Tours boat sometimes goes to the island for a $30 a day party trip.