We were just there in May and the Riverwalk is actually right at the cruise port. You can drop your luggage off and if it is really busy you can wonder around the riverwalk and shop or window shop to kill time. That is what we did and we didn't have to wait in a line up at all. It is a very nice port to cruise out of.
You can actually go on this web site and catch your cruise in port it's kinda neat.
Have a great time.