Vietnam has an enormous number of rivers that are at least partly navigable, but the most important by far is the Mekong and its tributaries. Scenic day trips by boat are possible on rivers in Hoi An, Nha Trang, Danang, Hue, Tam Coc and even HCMC.
Boat trips are also possible on the sea. Cruising the islands of Halong Bay is a must for all visitors to northern Vietnam. In central Vietnam the lovely Cham Islands (accessed from Hoi An) are a good excursion, while in the south, trips to the islands off Nha Trang and around Phu Quoc are recommended. It's now possible to reach the Con Dao islands from the Mekong Delta on a fast boat connection, too.
Note many boat trips are seasonal and subject to weather conditions.