Literal translation
he wouldn't find water at the sea
Meaning
said of someone who can't find what's plainly in front of them
Explanation
Said of someone who searches without finding what's staring them in the face, or who's judged not too sharp: "you sure look maritime pine to me!" answers, for instance, someone wrongly claiming to identify a tree. The image, self-evidently vivid for people living by the sea, first served to contest what someone said about themselves by pointing to their contradictory appearance, before extending to any contested statement. The compassionate exclamation pecaire! (or its frenchified variant peuchère!) is often added. It's an ironic, amused verbal joust, never aggressive, often aimed at children who make language blunders.