Lucy Gilliam, PhD, and Captain Arjen van der Veen are water people—it’s their life’s work. She is an environmental scientist; he founded a green freight shipping company. But water is also their home.

Cheryl Fellenz, 81, swapped her home for a houseboat in 2009 to cut expenses. Fellenz lives on $4,996 monthly from Social Security and a teacher's pension. She said the houseboat gives her close ...