Scam alert! They sale you tickets for private VIP minivan between Sihanoukville and Kep through Anny Tours and Champa Mekong finally its local bus full of Khmers stopping in Kampot and not Kep. Then you have to take tuktuk by yourself : 15$. This scam cost me more than a private taxi : 45$ only. During the way we feel very insecure. I understand why the country is sell to China. Cambodian in the region cannot make business at all.
If they say they are going to Sunset side they are lying. There is only one company Buva that goes there and only once a day.
Don't take the slowboat to the island. You should book the speedboat. Must re-confirm your return ticket while you are on the island and try to be at the pier early to catch the boat. They are usually late and sometimes over load with passengers.
Unless you really like the smell of people throwing up around you, pay the extra for the fast ferry, especially if it's even a little bit rough! It's well worth the extra few dollars.
Has providing all tickets to another island in Sihanouk city
I bought tickets here to Koh Rong island, ticket price is $20 (to/return). The ferry is big but old, can carry up to 80-100 passengers. They also have many new ferries but small, can carry up to 40-50 passengers. You also can take this ferry service to other destination such Koh Rong Samloem, Koh Thas, and so on.
Fast. What else do you want?
I was there to catch my boat to Koh Rong Samloem island. It took about an hour for the trip.