Koh Rong is a gorgeous sandy island with the clearest water on the coast of Cambodia, reachable by boat from Sihanoukville. I loved the fact that I totally forgot what traffic noises were like, as I only heard the sounds of waves and rain drops during our stay (visiting in late May meant beginning of rainy season.)
We flew from Siem Reap to Sihanoukville with JC Airlines, a short one hour flight. This is not the cheapest way to get to and from as taxes and airport fees are pretty high, but we found this to be the most convenient way as the only alternative would be driving (8 hours plus due to roads.) My advice would be to book in advance to get better prices as there is only a few airlines offering this route.
We chose to stay at Sok San Beach Resort for its remote location and authentic Khmer villas. We organised our airport pick up at a cost of 30 dollars with them (this is possible to be done once at the airport for a cheaper price.) A bit over half an hour drive took us to the port were our speedboat was waiting for us, once again this was booked through our resort costing 20 dollars per person each way and the only way to get to this part of the island.
Choosing to stay there in the middle of our holidays in Cambodia was such a good choice, giving us a nice break in between all the sightseeing in Siem Reap . We did not have the best weather during our stay with scattered rains, cloudy skies and rough sea on all our 4 days, but it just forced us to relax and enjoy lots of local food! We still managed to exercise everyday at the outdoor gym, go for runs on the beach and swim (obviously not me!).
PS: A sad reality for Koh Rong Island and the rest of Cambodia for that matter is the grand amount of trash being disposed onto the ocean which is mostly plastic. Unfortunately on the remote parts of the Island it was very noticeable even though the resort keeps its beach super clean.