Boat?

irot13,
there is no boat in this film, but there is a floatplane, and no one falls off it. Unlike some of the others, I happened to like this film although the last ten minutes loses something. Possible spoiler here: Fay Bainter plays a very juicey role, but betrays her character in the end (the change is set up by Melvyn Douglas' character, of whom I am finding to be an interesting actor). As I look for more of Douglas' films I hope to run into your "brother falling out of the boat" one.

- JKHolman

I feel fairly certain that would have been 'Captain Courageous' (1937) where a boy falls off a boat. Only Melvyn Douglas takes the part of the boy's father in that movie.