There are some boats between the mainland of Vietnam and the islands. Rach Gia to/from Phu Quoc takes 6 to 10 hours and costs around VND 44000. Officially, it should leave every day at 8 am in Rach Gia and at 10 am in An Thui. In reality it waits until there are enough passengers. This can mean day...so be prepared.
Ha Tien to/from Phu Quoc This is officially not allowed, since the boat crosses Cambodian waters so if you catch one of these the risk is your own as to whether you get put behind bars for your efforts or not.
Vung Tao to/from Con Dao There is a boat between Vung Tao and the island Con Dao. One way takes about 13 hours.
