It’s a hut.
https://donsmaps.com/lamouthe.html
Too bad, it’s clear that there have been watercraft for at least 800K years.
The cave may be too good to be true, regardless:
https://translate.google.com/translate?hl=en&sl=fr&u=https://fr.wikipedia.org/wiki/Grotte_de_la_Mouthe&prev=search&pto=aue
