We are most likely going to do the Galveston to San Juan as we just did it earlier this month and loved it. We did hover have 3 sea days - which is why we loved it so much. You may not be able to book the cruises back-to-back because of the Passenger Vessel Services Act (PVSA) which prohibits cruise lines from doing back to back delivery of passengers that do not stop in a distant foreign port. There is an exemption for transport between mainland US and Puerto Rico, but I do not know if it goes the other way. Your best bet is to call
DCL and ask. If you do - please post what you find out.