I'm not sure I'm applying logic. I'm just making an observation. Can you think of any successful ferries in the US that don't meet the criteria I outlined?

I'm not an expert, but my instinct tells me that people would be more likely to take a passenger train to Cape May for 3 reasons: 1) it could potentially be faster than driving, 2) it could potentially serve points along the way, like Vineland if it follows the highway, or other shore points, which would give other sources of traffic, and 3) it would go downtown-->downtown, which would be much more useful for carless shoregoers, who after a couple of hours on a boat probably wouldn't be in much of a mood to monkey around with jitneys.