No, it’s a colloquialism that refers to sedans as large as a boat. Of course boats come in all sizes, but it’s usually referring to something the size of, say, a present-day Chrysler 300 or larger. A truly full-size four-door sedan.

Also known as Land Yacht when they are also a luxury sedan: https://en.wikipedia.org/wiki/Land_yacht_(automobile)

This is a tongue in cheek example of a boat car: https://www.youtube.com/watch?v=w44qGwA1t58

Real-world examples: https://en.wikipedia.org/wiki/Lincoln_Mark_series