John and Melody Hennessee believe that their life on a cruise ship could be up to half as expensive as their life on land. They intend to cruise for the remainder of their days. The Florida-born couple, who bought a motorhome to tour the country, sold nearly everything they owned three years ago, including their primary residence and place of business.But later, according to Melody, 64, John, 76, grew weary of driving. Their quest to a permanent life at sea started when they came across an advertisement on Facebook for a 274-day cruise with Royal Caribbean.As part of their most recent extended voyage, the couple is presently touring the Dominican Republic; previous voyages have taken them to Australia, New