My guess is that they would have to permanently seal off any vent openings so that they couldn't sink/dive, even if they tried. It would truly have to be a surface craft. I think they could easily find the people willing to pay the money to motor around on a WWII diesel sub.
I was on the John Brown liberty ship when it visited Solomons Island, MD in 2004, I think. I visited the engine room and it was pretty impressive.