Most of these cards are real photo picture postcards. I suspect that they were created by enterprising photographers and sold to returning soldiers. They are almost always found unused. This is probably because they were bought and kept more as souvenirs, than as postcards to mail. Recently, I've seen some 'ship that brought me' postcards that, while based on photographs, are printed picture postcards, and not RPPC's.
Note that soldiers were given Jewish Welfare Board postcards, so they could notify their families that they had arrived. These could be mailed postage-free before arrival or on the dock.
I know little or nothing about the photographers and publishers who created these cards. The only consistent feature of these cards is lettering placed in the negative by the photographer, to the effect 'The Ship That Brought Me Home' or 'The Ship That Brought Us Home'. Note that these images were also sold as full-sized photographs, some even as panoramic photos.
In the following listing, it is noted if a JWB card is known for this ship. Some of the ships shown here are listed as having a JWB card, but I have not seen any example. Notations are similar to those for the JWB listing.