Edit: For non-cruise trans-Atlantic crossing by sea, the best I've been able to find on google is the Queen Mary 2. That seems to be generally considered an ocean liner rather than a cruise ship.
"The Italian Line's SS Michelangelo and SS Raffaello,[4] launched in 1962 and 1963, were two of the last ocean liners to be built primarily for liner service across the North Atlantic. Cunard's transatlantic liner, Queen Elizabeth 2, was also used as a cruise ship.[3] By the early 1970s, many passenger ships continued their service in cruising. By the first decade of the 21st century, only a few former ocean liners were still sailing, while others, like Queen Mary, were preserved as museums or floating hotels. After the retirement of the Queen Elizabeth 2 in 2008, the only ocean liner in service was Queen Mary 2, used for both point-to-point line voyages and for cruising."
http://en.wikipedia.org/wiki/Ocean_liner#Decline_of_long-dis...
(Of course this is just trivia and should not distract from the real point, which is that air travel has no real competition for a great number of routes, and the US government is increasingly restricting other forms of travel anyways.)
At the same time, isn't crossing the border firmly established as a "privilege"? In the good old days (ie a decade ago) you could just hop across the border pretty freely, but today you are required to have a passport or enhanced driver's license. Even fishing on the other side of the river (something my dad probably did at least four times a week when I was a kid) now has ludicrous reporting requirements, like calling customs via a landline to report you've crossed the border.
In any case, I've been researching trans-Atlantic passage and the QM2 seems to be the only way to get from the USA to Europe and back via water on a semi-regular schedule and not making a dozen party stops on the way.
Not a core source of revenue, but possible for someone needing to travel without using planes.
You are traveling on a working ship, with a very small crew, and not a lot of provided entertainment,(or internet access), so be prepared to look after yourself during these passages
According to that page, ships must hire a doctor if they have more than 12 passengers, so it's just not worth it for most of them to carry more.