Hi, we are going on the Nov 7th cruise and have a car reserved on all three islands. The rental place gave us an 800# to call from the dock and they will send a shuttle to get us. Not sure how smoothly it will go, but they told us to allow about one half hour. Hope that helps a little.
We always use Alamo. They have a shuttle waiting at Kauai and Honolulu. We called from the tender pier in Maui (be sure to book the WEST airport location if you are stopping in Lahaina). There is no charge for the shuttle in these ports, but there is in Kona (much further to the airport). Although the office in west Maui closes early, you can arrange a late drop-off and take a cab back to the tender dock (about $15).
We just book on-line at their website, then call to confirm with the local office on each island that our reservation is for a ship's dock pick up. Very simple.