Seems like I started a hot topic. Let me explain my perspective on this. I am a professional healthcare provider, so I have a bit of knowledge about germ transmission. Yes, norovirus can be found many places, and is definitely seen in ERs and clinics, but if you track these sorts of things, as does heath departments and CDC, it is not likely that you will find a large percentage of the population that has been affected at one time.
The concern I have is two-fold. First, when a large percentage of a population is affected by one particular organism, it is considered and "outbreak" (per CDC), and it seems as though this is exactly what occurs on these cruise ships. 10-20% of the ship's population is a huge percentage. (CDC's website specifically reviews this cruiseship virus.)
Second, the cruise ships know very well that this is occurring, thus they do have some responsibility for preventing it. (Consider all of the safety measures they already have in place - muster drills, tall railing, etc.)
Clearly, hand washing on the part of the passengers is essential, but what else are we expected to do?
Does the cruise line announce when an outbreak occurs so that the passengers take additional measures? Do the cruise ships do a "terminal clean" on the ship after the outbreak? Do they bring on extra staff to help with additional cleaning? What about more help during the cruise when the staff are ill? Have they been assessed for mode of transmission? Do they increase their food budget so that trully fresh food is put out for every meal - or is it recycled meal after meal? Specifically, how is the virus being transmitted from one stranger to another?
The norovirus is not like a bad storm, political unrest at a port-of call or general sea-sickness. These are not things that the cruise lines have any control over. The transmission of a known present virus may be.