There are no car rental places near the water taxi terminals. I recommend calling ahead to Crystal Auto rental. They will pick you up at the water taxi and take you out to their lot. Crystal is up the Northern highway about 3 miles. From Crystal you can go north and then take the Boom road that meets the the Western Highway at Hattieville (and thence to Belmopan). This route is far easier than navigating through Belize city.