The Phu Quoc Island (or Koh Tral as the Cambodians call it) lies 15km offshore from Cambodia and 45km west of Ha Tien and is full of mountains, forest and endless white sandy beaches. To travel from Kep to Phu Quoc Island, you can take bus to Ha Tien,a small town very near the Cambodia-Vietnam border, and from there, take a speed boat (1,5 hours) to Phu Quoc.
 

 

commented Jul 14, 2015 by Sunshine (2,380 points)

These information I collect from blog of Tom, an independent traveller in Vietnam since 2005 till now. You can check more here http://vietnamcoracle.com/phu-quoc-island-ferry-times/
PHU QUOC FERRY TIMES & FARES
The following times and prices are correct at the time of writing (March 2014), however, things are set to change (for the better) when two new ferries join Superdong’s fleet later this year. Check the Superdong website (http://www.superdong.com.vn) for the latest information (although you might find it better to call, as their site is rarely updated). Keep in mind that, because the pace of development on the island is so rapid, some details may have changed by the time you read them.
Note: times given in brackets are additional sailings laid on during high season (December to March) and/or when scheduled ferries exceed their capacity.
RACH GIA – PHU QUOC (fast boat)
Departures: 8.00am (9.00am) and 1.00pm (12.40pm) daily
Duration: 2.5 hours
Passenger Ticket: 350,000VNĐ ($17)
Motorbike Ticket: 250,000VNĐ ($12)
 
PHU QUOC – RACH GIA (fast boat)
Departures: 8.00am (9.00am) 1.00pm (12.40pm) daily
Duration: 2.5 hours
Passenger Ticket: 350,000VNĐ ($17)
Motorbike Ticket: 250,000VNĐ ($12)
 
HA TIEN –PHU QUOC (fast boat)
Departures: 8.00am (8.30am) 1.00pm (1.15pm) daily
Duration: 1.5 hours
Passenger Ticket: 230,000VNĐ ($11)
Motorbike Ticket: 180,000VNĐ ($9)
 
PHU QUOC – HA TIEN (fast boat)
Departures: 8.00am (8.30am) 1.00pm (1.30pm) daily
Duration: 1.5 hours
Passenger Ticket: 230,000VNĐ ($11)
Motorbike Ticket: 180,000VNĐ ($9)
 
HA TIEN – PHU QUOC (slow car ferry)
Departures: 8.20am (1.15pm) daily
Duration: 2.5 hours
Passenger Ticket: 180,000VNĐ ($9)
Motorbike Ticket: 100,000VNĐ ($5)
Car Ticket: From 900,000VNĐ ($45)
 
PHU QUOC – HA TIEN (slow car ferry)
Departures: 1.45pm (8.00am) daily
Duration: 2.5 hours
Passenger Ticket: 180,000VNĐ ($9)
Motorbike Ticket: 100,000VNĐ ($5)
Car Ticket: From 900,000VNĐ ($45)