There are 2 car rental places within walking distance from the ferry, Affordable & Young's Bicycle Shop (despite the name, they rent cars too) You don't say what time you are arriving but it appears Affordable closes at 5 pm & Young's at 6:30. Not, it is not absolutely necessary to rent a car as there are taxis and the NRT Wave bus system but you are far enough out of town that I would suggest it. You'll get to experience a lot more of the island and if you need groceries, etc it is easier and probably more cost-effective with a car.
Thank you very much for the information
The airport also has several car rental places; if you decide to rent from one of them (we found a large difference in prices the one year that we rented a car), you can get a cab from the ferry to the airport.