Houston to Galveston takes approx. 1 hour. Shuttles are offered from many hotels.
As far as Montego Bay goes, only do an excursion offered through the cruise ship. Although beautiful geographically, Jamaica is a rough country with not much policeing.
We enjoyed Dunns River Falls (Ohos Rios) immensely. Climbing the falls is strenuous and for people who are in pretty good shape. But, it's well worth it! Have fun and stay safe in Jamaica!