It looks like this motorboat is registered to Austria. Flags at the back of the boat or rather rear rudder-engine area are simply known as the rear ensign, which denotes the national registry of the boat - kind'a like a license plate.
The flag of this boat is a horizontal tri-bar of black-white-black. It looks like the colours of Prussia but Prussia was no longer around, at least officially in the 1930s.
The British Red Ensign made an appearance in this movie, during the boat chase. The British Red Ensign has a red field with the regular UK flag in the upper left corner - also known in flag jargon as the canton.
The Kingdom of Italy flag also appears briefly in this picture. The main difference between the modern Italian Flag and the Kingdom Flag is a coat of arms. The Kingdom has a white cross on red field with a blue border.
A middle eastern flag with a red eight pointed star trimmed in yellow on a green background.