If you only need it to book the car, then go with that. However, river boats are given their docking spots by the local port authorities (maybe not the right name - but someone has the job to do this...) and the actual spot may not be known until the day of docking.
How are you getting from the car rental spot to the boat? If you are taking a cab, I would suggest calling the boat at the time you drop off your car (the number of the boat will be in your final documents) so you know exactly where the boat has docked.
Fran