Houston and Galveston are on average an hour from each other depending on were you are in Houston and traffic. Right now there is no one sailing from the new Houston ship terminal although we are hopeful that NCL will be there next year and hopefully MSC as well.
The airport is the weak link for sailing from Texas. It can be an adventure getting from the airport to the ship terminal in Galveston. Depending on the number in your party it may be best to just use the ship transfers.
It's really an easy port especially for locals and Texans who drive in such as myself. The weather is usually good and the port and cruise staff are very friendly. A lot of people who fly come in a day early. Galveston is an interesting place to visit. At debarkation, I'd recommend late flight departure and take the NASA excursion which will then drop you at the International airport. It only a few $ more than the shuttle and at least you aren't rushed and you get a tour.
Of course IMHO the biggest advantage of sailing from Galveston is that there are lots of Texans on board ship.
They are a fun group!