I think at one time they were not valid in law, but since now you can get married anywhere that holds a licence issued by the Registrar General then it will be legal. Places like Football grounds and hillsides have all been licenced. Whether the registered port of the vessel counts I am not sure. http://www.aweddingforyou.com/cruiseship.htm
this has the rules
The Captain also has to be a registrar..if he is *just* the Captain he cannot legally perform the ceremony. You can marry on a cruise ship as long as the vessel as a licence..