The one problem I would see is that the ports are usually right in the middle of cities (big cities). I would hate to take off on a bike in some of the cruise port cities I have seen. In fact, it is busy traffic for even some of the cabs I have been on getting off the ships. You would never pick these roads for a bike ride in any other circumstance.

I have about 10 Spinervals and CTS videos on my Ipod touch that I use on the cruise ship spinning bikes to keep my legs in shape (and to wear off the cheesecake). Then I enjoy the ship and ports the same as everyone else.