Japanese safety slogan

Your understanding of "one-person tour" is not wrong, and so is the requirement of accurate grammar.

However, the slogan on people's lifebuoys is also the most standard Japanese expression.

....は.....です

People swim with nouns as the subject.

なぃです adjectives and auxiliary verbs together form the predicate.

See if that's what your grammar book says

In addition, the slogan is generally concise, and the landlord's sentence is "one person, one person, one person, one person, one person, one person, one person, one person, one person, one person, one person, one person,