Article 46 of the Geneva Convention makes specific mention of this though:
"The Detaining Power shall supply prisoners of war during transfer with sufficient food and drinking water to keep them in good health, likewise with the necessary clothing, shelter and medical attention. The Detaining Power shall take adequate precautions especially in case of transport by sea or by air, to ensure their safety during transfer, and shall draw up a complete list of all transferred prisoners before their departure."
I can't think of a case where anyone was ever prosecuted for failing to properly mark a prison ship that was subsequently attacked though.
Some of those associated with the Japanese hell ships were executed (and others imprisoned); but not properly marking the ships which were sunk in air or submarine attacks were among the least of their crimes. I do note that in one case, as here, the captain was determined to be innocent of the wrongdoing.
CougarGA7: "Not marking a prison ship is a war crime. "
CougarGA7: quoting from Article 46 of Geneva Convention:
"The Detaining Power shall take adequate precautions especially in case of transport by sea or by air, to ensure their safety during transfer..."
From just this, I'd say any claim that Brits committed a "war crime" is based solely on anti-British animus.
The Geneva Convention does not here spell out what "adequate precautions" consist of.
Surely it would be entirely "adequate" to provide as much protection for those prisoners as the Brits provided their own sailors?
So I'll put it in the form of a challenge: can you cite a country, besides the United States, which treated its prisoners better than Brits treated theirs?
Yes, I think we can say that Brits are sometimes risk takers, and less concerned with safety than we Americans think appropriate (again cite the Gulf of Mexico oil spill, for recent example).
But that is a far cry from calling them "war criminals," imho.