"romantically desirable person," 1947, from dream (n.) + boat (n.). The phrase was in use about two decades before that. "When My Dream Boat Comes Home" was the title of a 1936 song credited to Guy Lombardo and "Dream Boat" was the title of a 1929 book.
It is rare indeed that a designer ever has the opportunity to build his dream boat. Chris Smith, all his life, had held in mind a boat of about fifty feet overall which would be the last word in yacht design and performance. [Motor Boating, December 1929]