From the pier a couple of km west of My Tho, occasional freighters leave for the town of Tan Chau which is the closest town to Chau Doc on the Mekong River. If you want to travel to the Cambodian border by freighter, this is the way to go. We were told departures were a few times a week with the trip taking two days. Fares varied from 70,000D to 150,000D (for the same boat!) so ask around, bargain hard and pack a hammock!
There is a small ferry pier on the canal that runs by Trung Trac St. From there you can catch boats to Dragon, Tortoise and Unicorn Islands.
My Tho is small enough that you can walk to any of the points of interest -- the furthest out of town, Vinh Trang Pagoda is about a thirty minute walk from the riverbank. Otherwise there's the occasional cyclo (keep an eye out for Mr Nip -- he'll most likely find you -- a cyclo driver who speaks excellent English and is a fountain of knowledge about My Tho) and xe-oms about.