I had to image-search for both... but indeed I find the equation of the two strange, especially because of the water. As for more local cultural associations: in Germany (at least around Frankfurt), birkenstocks were somehow associated with old people. Not so in Hungary (though that may have changed since -- I rarely see them on young people in the summer, flip-flops spread here too).

_{*Lunatic*, n.
One whose delusions are out of fashion.}