Actually, to go to the Koh Rong island or Koh Rong Samloem island from Sihanoukville mainland it needs everyone to get ferry tickets of your self and for your friends.
To Go to the island as you already planned ‘to be on time’ then it need  you to book the speedy boat ticket ahead before the departure time, sometime it make you disappointed because of the number of the passengers are limited
To kick off the disappointed then let get the speedy boat before arrival time, you can talk to an agency through online by calling them
On the other hand, Cambodia Shooting Ranges is able to help you to get the your speedy boat ticket on time, please free let us know what time and departure date
The group of Cambodia Shooting Ranges Outdoor Phnom Penh is also can arrange the shooting activities, private taxi and ticket to go to the island, just contact through email @ CambodiaShootingRanges@Gmail.Com WhatsApp and Telegram @ +85578777908

Here is the daily boat schedule to go to Koh Rong and Koh Rong Samleum island are bellow;