As I recall, there were rental agencies in every port we disembarked at. But you save money if you do it in advance. Yes, I do believe they will have cars waiting for you at the port, and this is the USA so all the biggies are there. I would just look online for the best prices and call them to verify.
I must say, though, that we rented a car in Oahu and got lost, stuck in traffic, and didn't see anything good. Then when we returned some people we met drove us around, and it like to two completely different islands.
If you DO rent a car, pre-plan your route. The islands have small roads and LOTS of cars. You can easliy get lost or stuck in traffic and waste hours. Doing shore excursions is notr so bad by comparison, or hire a private driver.