Siem Reap is a nice place to go, from there you can easy take busses to other places like Bangkok Phnom Pehn, Sihanoukville and Kompot.
I took the nightbus (depart at 19.00) from Siem Reap to Sihanoukville. It is an 12 hours drive on bumpy roads and the beds are not that big. But I heard a tip. Just eat a small happy pizza before you go and you’ll sleep like a baby.
When you’re arrived in Sihanoukville there you have to go to Serendipity street. Thats the place where the boats depart to Koh Rong. When you get out of the bus there will be tuktuks ($4) motor taxi’s ($2) or guys who want to sell you a boat tickets and bring you for free to the boat. (you will arrive at 7.00) the first boat departs at 8.00.