You know, I never thought about that before...

[garygnu]

@Quercus

Nice try, but no plot-hole closing here. As wiki says, “The first boat to be fitted with a snorkel was U-58 which experimented with the equipment in the Baltic during the summer of 1943.” No 1937 boat had a snorkel.
But they did usually stay on the surface unless they had a reason to dive. And probably fewer reasons to dive in 1937 than during real war time.

I stand corrected. Revised statement: “WW2 submarines generally stayed on the surface while cruising.”
Maybe I should have read my own link. :smack: