How to get travel Siem Reap
The Phu Quoc will ferry let you off at Ha Tien. There is a canal linking Ha Tien to Chau Doc. I guess you could hire a boat from Ha Tien to Chau Doc, don’t know. From Chau Doc, boats go everyday to Phnom Penh. From PP, boats travel daily to Siem Reap. So, it might be possible to go from Phu Quoc to Siem Reap, all via water. Tours on Indochina
All the boats depart at 7.30 am. There is no afternoon boat. Also, there are public boat from Chau Doc to Phnom Penh and then from Phnom Penh to Siem reap. The price is US$ 50 for each route.
From Phu Quoc to Ha Tien, and from Ha Tien to Chau Doc, you must charter one boat. I am not sure about the price but it is very expensive. I recommend you to take the flight. It helps you to save lots of time.
If you take a boat from Phu Quoc to Siem Reap, you should spend several days. Please imagine that it takes 5 hours from Phu Quoc to Ha Tien, 5 hours from Ha Tien to Chau Doc, five hours from Chau Doc from Phnom Penh, 6 hours from Phnom Penh to Siem Reap.