The second stop in your Royal Tombs tour usually is the Tomb Of Tu Duc. Your boat will stop on a dock by the river, you will climb some steps and get to a road. On the road you will find a bar, and some local guys with motorbikes. After negotiating the price of the ride, they will take you on their back seats to the Royal Tomb. They will wait for you outside the temple. Usually you pay once you get back to your boat.
Unfortunately I did not have enough time to visit the tomb in detail, but I did enjoy my visit.