Garry Hubbard didn’t envision a quiet life for himself and his wife, Libby Hubbard, on solid ground. He wanted to retire aboard a yacht.

With the help of Phil Friedman, managing director of the yacht consulting firm Port Royal Group, the Hubbards spent four years and roughly $5 million to design and build a retirement home that would take them from the Texas coast to Florida, and from the Bahamas up the eastern seaboard.

The...