Many cruise ships have oyster bars or serve oysters but they can be extremely dangerous to your health.
There is a bacterium called Vibrio which is found in oysters among other things and causes three different diseases in humans which are Vibrio cholerae, Vibrio parahaemolyticus and Vibrio vulnificus.
Back in 2004, there was an outbreak of Vibrio parahaemolyticus on an cruise ship in Alaska which resulted in 62 out of 189 passengers becoming sick and all of the oysters on board were found to be infected. They had come from an oyster farm in Prince William Sound and over years the water there had been getting warmer which meant that it was now sufficiently warm for the bacteria to grow.
Throughout the world, each year Vibrio cholerae causes three million to five million cases of cholera and 100,000 to 120,000 people die. Most of the infections are in Africa, in fact in the United States there are only between five and 10 cases of cholera each year.
Vibrio parahaemolyticus is more widespread and causes about 4,500 infections in the United States annually and usually results in diarrhea. Luckily deaths from this are rare.
The much more serious problem is Vibrio vulnificus which causes around 95 cases in the United States each year resulting in almost all patients being hospitalized and 35 deaths. It is often described as the most virulent bacterium in the world with a 50 percent fatality rate. It causes what is called necrotizing fasciitis, also known as the flesh eating disease and 90 percent of all of the cases in the United States came from eating raw oysters from the Gulf Coast. If a healthy person contracts it they usually just have a mild case of diarrhea but those with wound infections, or immune disorders will often develop septicemia which then turns into the flesh eating disease.
So before eating oysters on board a cruise ship, especially if you do not know where they came from, think carefully about the risks.