Interestingly, the French Canadian folk-rock band Lougarou, whose name was a homophone for loup-garou ("werewolf") had to change their name to Garolou, which was a homophone for "gare au loup," beware of the wolf.

Using the same syntax as the dictionary maeve linked to earlier, this would be rendered as "Ware Wolf!" So they went from "werewolf" to "ware wolf" without even trying!

Seriously, to most french-speakers, Gardez L'eau would not make sense in the context. It means, more or less, "keep the water." So I think Billy's etymology is close to the correct one (as does maeve's dictionary).

Billy is not entirely correct as to why, however. The language changes in 200 years don't come into it. Today, as then, "gare" means "watch out," "beware." Nowadays to say "beware of the water," you'd be more likely to say "gare a l'eau," than "gare de l'eau," but people also use "se garer" as a verb, in which case it takes "de." So "gare de l'eau" is not so big a stretch in today's French...