where are we supposed to intern prisoners, other than in a prison?

From here:

Art 22. Prisoners of war may be interned only in premises located on land and affording every guarantee of hygiene and healthfulness. Except in particular cases which are justified by the interest of the prisoners themselves, they shall not be interned in penitentiaries.

Prisoners of war interned in unhealthy areas, or where the climate is injurious for them, shall be removed as soon as possible to a more favourable climate.

The Detaining Power shall assemble prisoners of war in camps or camp compounds according to their nationality, language and customs, provided that such prisoners shall not be separated from prisoners of war belonging to the armed forces with which they were serving at the time of their capture, except with their consent.

To be fair, I'm not entirely sure how the US is in contravention of this article. I assume that the proscription of penitentiaries is designed to prevent POWs being locked up along with criminals -- which would imply that they were criminals.

Moreover, it specifically states that "The Detaining Power" should build camps for the prisoners.

About the only thing in here that I'm not sure about is whether the "climate [in Cuba] is injurious for them".

Edit: The introduction to the Geneva Convention is here