Highly recommend that you rent a car at Hobby and turn it in the next day in Galveston. We used Enterprise and this worked very well (from the Galvseston pier to Hobby airport)
Another idea is to spend the night at a Hobby airport hotel and take a shuttle such as Galveston Limosine to the Galveston pier. It is about an hour- hour and 15 minutes from Hobby to Galveston.
Hope that this helps.