What Merchanht ships were declared as "protected" in WWII for the transferred of Prisoners of War, between warring nations
2Are you asking what types of ship were used or are you asking for actual, named examples of the ships that were used?– Steve BirdMay 15, 2016 at 8:13
What evidence have you found to support the notion that ANY such ships existed. Submarines don't stop and ask for identification papers before firing torpedoes.– Pieter GeerkensMay 16, 2016 at 0:14
There is a book "Mercy Ships: The Untold Story of Prisoner-of-War Exchanges in World War II" which covers this topic.
The two principal ships used were Swedish: Drottningholm and the Gripsholm.
These ships were also used to exchange civilian internees. The total amounts of people exchanged amounted to a few 10s of thousands. In the case of soldiers, it was mostly sick or disabled soldiers that were exchanged.