We faced a similar situation a couple of years ago when we cruised out of Galveston on the Grand Princess. Since we wanted to fly-in a day early (we always do this in order to improve chances of having our luggage, to relax, and not worry about missing the ship) we decided to rent a car from Enterprise. They were very cheap (cost us less than $45..even with the one-way rental) and they have a dealer about a mile from the port who drove us to the ship after we dropped our car). You might be able to rent a large van that would hold your entire party. One advantage of going a day early was that we got the bonus of enjoying an afternoon and evening in Galveston.