Mister Magoo's Puddle Jumper (1956)

At a used car lot, Mr. Magoo is intent on buying a car for his nephew Waldo. He is slick talked into buying an old clunker thanks to a shifty salesman but he drives it off anyway. Unfortunately, the myopic Magoo drives off a pier and under the ocean where he mistakes the various aquatic surroundings such as fish, sunken boats, and seals for other cars, dilapidated mansions, and a horn happy driver respectively.