What Am I?

This coconut-like pod washed up on the beach today. It's about the size of a softball and not very heavy. It's rubbery (not husky like a coconut), but that could just be because it is so waterlogged.

Its I.D. confounded some older, wiser, and more widely-traveled beachcombers than the Current's staff, though we all agreed that it's vegetable in nature.

Any idea?