Boats can be hired from near the small bridge on the Ben Tre river or via the Ben Tre Tourist Authority and if you're in Ben Tre this is a well worthwhile trip.
Prices vary somewhat -- expect to pay around 100,000 VND per hour if arranging privately (more if via a hotel) and the boatman will want you to do a longer trip of 3-4 hours. While there is nothing wrong with spending that long, you really only need two hours to see the best parts.
We particularly enjoyed a section of very narrow canal close to Ben Tre and a more expansive, winding canal on the far side of the Ham Luong River. We found this to be an especially beautiful area rich in scenery and river life.
When we crossed the Ham Luong there were a number of medium-sized freighters moored midstream that were being loaded -- by hand -- with fresh cut coconuts. There was an awful lot of coconut-chucking going on.
The boatmen tend to try and pad out the time with a trip to some Taiwanese-owned fish farms, but they are really not worth the effort -- nor the extra 100,000 dong that you'll incur to do so.