Koh Rong is located about 25 kilometers from Sihanoukville, in the Gulf of Thailand. The island has 43 km of beautiful sandy beaches along both its Eastern and Western shores; its main attraction. Most visitors stay in guesthouses and bungalows situated along the beach at Koh Tuich Village (Tui Beach). There are isolated resorts on other beaches. The atmosphere at Koh Tuich is very relaxed and beach-focused during the daytime, followed by beach restaurants and partying when the sun sets. Travelers looking for isolation or peace and quiet should consider accommodation at the far end of Tui Beach or on other beaches.
Koh Rong has two ferry options. The slow boat leaves 2-3 times daily and takes 2 1/2 hours. ($5/10$ round trip with open return as of February 2014). The fast boat takes 40 minutes-1 hour and leaves about twice daily. ($20 R/T with open return). Two of the fast boat companies have websites, these are Speedferrycambodia and Sea Cambodia, the websites have up to date information on destinations and timetables.
The ferries are fairly basic and not very comfortable. The new high speed ferries are pretty comfortable, but often times more tickets than seats are sold, so make sure to show up early if you don't want to stand for the hour long journey. Make sure you carry a hat and sunglasses for the ride, and use sunscreen if required. As of Feb 2014, water is not sold on the ferry and neither are snacks or other drinks. It may be a good idea to carry some with you, especially if you are travelling with children. There is a small toilet on board the ferry which seems to be clean and in decent working condition.
To board and disembark from the ferry you will need to walk on an average sized plank of wood connecting the pier to the ferry. The process is not very efficient and can be a little scary the first time you try it, so bear this in mind if you have any foot injuries, have special needs etc.
There seem to be at least two different slowboat services, so shop around the agencies in town to see what they offer.
1) The Dive Shop and Monkey Island ferries leave Sihanoukville in the morning and in the afternoon, at varying times: the morning service is between 8 and 9 am, and the afternoon one between 1 and 2 pm. The Boat is returning at 10 am and 3-4pm from the island to Sihanoukville as of May 2013. This ferry will not take you to Sok-San bungalows, which is on the other side of the island (Long beach). These ferries take around two hours. There is also now a fast ferry taking any 40 minutes and traveling four times a day. The fast ferry costs $20 return.
2) Similarly, Sok-San bungalows run a ferry service which leaves Sihanoukville at 9 am or so, but this ferry will not take you to the side of the island where the majority of guesthouses and tourist facilities are situated.
Make sure that you are absolutely certain that the ferry you are booked to take will drop you off at your destination. There have been instances of resort owners refusing to let guests take boats to the other side of the island (Sok-San to Monkey Island etc.), and insisting that you stay on their beach. Also, if you are on a schedule make sure that you arrive at the pier for your return as the ferry is first come first serve and there isn't always space.
Also note that all ferry tickets include a ride to and from the main town area in Sihanoukville, so you should not have to pay extra for the same. The buses from the city centre leave at least 30 minutes before the ferry departure. Sometimes, though, the ferries leave from the main pier on the Serendipity beach.
There are no roads on the island yet so the only way to get around is on foot. The walking man of Koh Rong (an Israeli guy called Gil) has marked trails that lead from the beach closest to Monkey Island and Paradise Bungalows to other beaches but most of the island is still thick jungle. He has since left. Some other guy at Paradise Bungalows supposedly offers trekking tours, but not to the beaches and charges $20. The main well-known hiking path is the one to Long Beach (also known as 7KM Beach, Sok San Beach). The only other option is to hire a local fishing boat to take you around the Island. Be advised that nowadays guesthouses / hostels will lie to you saying the walk to Long Beach is "too dangerous" (which is not true!) so that they can take you there for $10 on a group boat trip that they all offer every day which includes snorkeling, fishing and mediocre BBQ.
With 23 white sand beaches to choose from Koh Rong has plenty to see and do. For the adventurous there's plenty of trekking to be done through the relatively untouched jungle. Diving and snorkelling is excellent both off the beach and at sites nearby. But above all the best attraction is the phosphorescent water which sparkles when disturbed like twinkling stars. Best seen in around 4 feet of water and on a dark part of the beach. Because Koh Rong has minimal development and light pollution the conditions are perfect for the phosphorescent plankton and it can be seen every single night.
Be sure to head to Long Beach, either by foot through the jungle or by long tail boat. The hike is 45 min-55 min each way, through dense jungle and lots of scrambling on steep wet rocks closer to Long Beach. Lots of mosquitoes. Bring water (please don't litter/sad to see so much rubbish). As of July 2015: Long Beach isn't the glorious beauty that was expected due to massive construction on the left side of the beach. Also filming Survivor on the right side of the beach. Perhaps by high season (Nov-Feb), there will be new accommodations after construction finishes and the film crew will have departed. To get there: find one of two entrances from the main Koh Rong Beach , either through the alley near Dream Catch Inn, or from Sky Bar. Then turn LEFT on the trail and you will see periodic red trail markings.
Also be sure to check out HIGH POINT ADVENTURE PARK, this is the newest activity on Koh Rong and has without a doubt the best views of the ocean, the jungle, and the main beach on the entire island. Perfect place to take some very envy evoking pictures of this incredible island
Snorkelling. Various places and people will offer snorkelling trips on their long tail boats. Native Khmer guides, while cheaper, will most likely be unable to communicate with you in english and will just point to things. $10-$20.
If you're a diver, you can Scuba Dive on Koh Rong, and with over 15 dive sites less than 30 mins boat ride from the center you can do multiple dives and see different things every time. If you are not a diver, then the Koh Rong Dive Center has a full range of teaching courses from Open water to Dive Master that you can do at one of the cheapest prices in all of Asia. Keep in mind during "low season" the weather is not very cooperative, and thus the waves are choppy, sand is mixed up, and visibility is about 1 meter or less (read: not suitable for diving).
You can also go from the depth of the ocean to the heights of the tallest trees on the island with HIGH POINT ADVENTURE PARK. This is the newest attraction on Koh Rong opening up on in March 2014. It has over 25 stations ranging from a 120 meter zip line to rope bridges to a surfboard zip line. HIGH POINT also has the best views of the whole island and is a must do while on Koh Rong.
Pub Crawl. Check the bars on the island. Meet some friends alongthe way. Makes for a good, fun, night out. Every Tuesday and Friday night.