The river cruise between Dartmouth and Totnes is around 90 minutes each way. It is operated by Dartmouth Steam Railway regularly between Totnes and Dartmouth, but the timing varies depending on the season, so make sure to check the website before travelling. Tickets can easily be booked online or if you're in town, you can buy them in person at ticket offices in Dartmouth and Totnes.  Buy tickets and find out more here