We actually did the falls/White River tubing when we went on our last cruise. We had a large group (39 people) that were together, so needless to say we weren't an "easy" tour group. I actually booked through shorexcursions.com but our guide was Denton Pendergast from Karandas Tours. I have kept his business card because he did such a fabulous job with our large group. His e-mail is firstname.lastname@example.org
or you may be able to go through shoreexcursions.com again. Good luck and have a GREAT trip!