Congrats Bem, on your cruise finally being here!
I would follow Cruise Fanatic's advice about the taxi from the airport. I'm pretty sure the Sheraton RiverWalk has a complimentary shuttle service to the cruise terminal(they used to). You should call them and verify though so you can remove that concern. If they still do, ask the Front Desk about the schedule and try to get on one of the earliest ones. The Legend will typically start boarding before noon, if everything is on schedule.
Have a wonderful cruise!