Mickey Mouse decides to name the ship he, Donald Duck and Goofy are building the Queen Minnie after his girlfriend Minnie Mouse, who christens the ship at the end of this cartoon. It's also a reference to the real famed ocean liner RMS Queen Mary, which was brand new when this cartoon was released.
"Boat Builders" from 1938 is the third Cartoon in which Mickey wears Overalls and the second time in which they are red. The first Cartoon (in which they are green) is "Mickey's Elephant" from 1936 and the second (in which they are red) is "Clock Cleaners" from 1937.
Mickey's Nephews Mortie and Ferdie, as well as Clarabelle Cow all make cameos at the christening ceremony in "Boat Builders" from 1938. The Theatrical release poster of the Cartoon also features Horace Horsecollar and Pluto.
"Boat Builders" from 1938 is the third of five Mickey Mouse Boat Cartoons, it also is the first Mickey Mouse, Donald Duck and Goofy Trio Boat Cartoons and the first of two in 1938, with three overall.