Once you've entered your vehichle of choice after leaving the ship (limo shuttle service or whatever), the drive from the port area in Galveston to Houston Hobby will be approximately 1 hour, perhaps *very* slightly less perhaps *very* slightly more, depending on traffic. Add in however much time you need to arrive at the airport before the actual flight out (I'd advise about 1 hour before boarding your flight -- Hobby's a small airport), and you can probably figure out your best schedule. My best guess, based on an early-morning departure from the ship (say, 8 a.m.) would be to safely schedule your flight out around noon or 1 p.m. that day, but probably no earlier than 11 a.m., depending on the ship's schedule.
Warning!!! This advice is based on a one-time cruiser's experience, but someone who has lived in the Houston-Galveston area for roughly 40 years.