A boat-tour around Ha Long Bay is one of Vietnam’s most enjoyable trips, while scheduled ferries sail year-round – weather permitting – to the major islands off Vietnam’s coastline, including Phu Quoc, Cat Ba and Con Dao. In addition, ferry and hydrofoil services run from Hai Phong to Cat Ba, and hydrofoils from Ho Chi Minh City to Vung Tau, and from Ha Long City to Mong Cai and Bai Tu Long. Though they are gradually being replaced by bridges, a few river ferries still haul themselves from bank to bank of the various strands of the Mekong from morning until night.
This is only really an option in the Mekong Delta, where you can travel in both tourist boats for short-haul trips and take freighters for longer trips. The former are comfortable, the latter can sometimes be comfortable, other times less so. Boat transport is slow -- figure on two days for a trip from My Tho to Chau Doc on the Cambodian border. The most popular tourist services are the ferries from Saigon to Vung Tau, and the boats from Chau Doc to Phnom Penh. Boat travel generally works out as being more expensive than bus travel over a similar route.