It may not be a myth!  I lived in northeast Florida for a few years.  The
beaches were gently sloped, and at low tide were easily a hundred yards
wide, of hard packed sand, quite capable of supporting a vehicle.  People
would drive down onto the beach, then go swimming or walking down the beach,
or whatever. Then the tide would start coming in.  You could see the waves
washing farther and farther up the beach, toward cars whose owners were not
in sight. Finally a wave of sufficient size would wash up far enough to
travel under a car.  As the wave receded, sand would be washed out from
under the tires, and the wheels would sink into the sand.  Once that
happened, that particular car was not going anywhere under its own power.
Tow trucks would actually patrol up and down the beach, looking for victims
they could pull out, taking advantage of a desperate situation to charge
exorbitant rates. Better to pay a hundred dollars for a tow right now than
to have your car submerged.  But they wouldn't pull out a car whose owner
was not there. Several times I saw a car out in the surf, half covered with
water, waiting for the return of its unfortunate owner.