Guest is correct if he's talking about extemporizing new, four-line rhyming stanzas of some merit, less correct if he means two-liners of no merit, and quite wrong if he means one-line verses that don't need to rhyme because they're doubled to fit the structure of the shanty.

Most shanties, as actually sung by men simply trying to get a job of manual labor done, fell into the two-line (or one-line) category. It is very easy to invent a nonrhyming, stand-alone line to (more or less) fit the meter. It's easier still to import a known line or a couplet from some other shanty, or to alter it just a little.

What many current singers don't fully understand is that at sea the shanty served solely to aid in the very hard work of heaving and hauling. Few sailors cared anything about musical or lyrical quality, any more than modern military recruits worry about the professional entertainment value of marching chants. Nowadays the beauty of a shanty comes from accomplished performance. At sea, the performance was rough and ready. The beauty came partly from the idea of the melody, partly from the setting and the weather, partly from the satisfaction working at a challenging task together, and partly from thinking about all these things later.

We're all lucky that the melodic quality of so many shanties is as good as it is, and that enough of the words remain interesting.