Koh Rong is an Island off the coast of Sihanoukville.
Koh Rong can be reached by a ferry ride from Sihanoukville. The ferry itself is pretty basic and there are no proper seats on any of the ferries. There are benches on either side of the ferry, leaving the middle portion to be used for luggage, though some enterprising travellers use the area to stretch out and catch a quick snooze. Make sure you carry a hat and sunglasses for the ride, and use sunscreen if required. The ferries usually leave in the morning to the island from Sihanoukville, and do a return trip in the afternoon, so it will be sunny. As of now, 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.
Currently, ferries run to two opposite sides of the island.
1) The Dive Shop and Monkey Island ferries leave Sihanoukville at around 12 pm or so and reach the island in the afternoon. There is also a return ferry running at 10 am from the island to Sihanoukville as of February 2012. This ferry will not take you to Sok-San bungalows, which is on the other side of the island.
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 Monkey Island bungalows, Treehouse bungalows, Paradise bungalows, Pura Vida, etc. are situated.
3) For an up to date boat schedule and price list visit the local Sihanoukville Travel Guide - SHVguide.com - It is operated locally and keeps info up to date
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.
All services cost $15 return (lately, most of the bigger boats started charging 20$ with return). However, if you find that you have ended up at a different place from where you landed on the island, you can still catch a ride on any of the return ferries if they have room. You will be charged $10 for a single trip from the island to Sihanoukville.
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.
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. Gil knows the hidden trails of the island very well and offers trecking tours from 1 to 4 days (cost ca. 10 USD per day per person and is worth every dollar). The only other option is to hire a local fishing boat to take you around the Island.
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.
Internet is surprisingly good on Koh Rong, near to the ferry pier at least. Apparently an optical fibre cable link has been recently installed. Many of the restaurants/bars/accommodation providers near to the ferry pier now have wifi.
Mobile phone reception is almost non-existent.