We think this is a young durian
fruit. These smell bad when mature and ripe, so we decided not
to find out what they smell like when they've been sitting in
salt water, in the sun, on the beach for who-knows-how-long.
That looks very much like an Osage Orange, except
they tend to be found in North America. Supposedly they smell nice and
repel mosquitos (or so my third grade teacher said), but since they
grow in warmer climates perhaps someone planted one in Uruguay?