Does it smell?
If so, have a look at ambergris and tell us what you think...
To quote from
"Freshly produced ambergris has a marine, fecal odor. However, as it ages, it acquires a sweet, earthy scent commonly likened to the fragrance of rubbing alcohol without the vaporous chemical astringency"
I sure it is ambergris; the earthy smell is right for an old piece.
However, I should ask: does it burn?
If so, it is either ambergris or paraffin wax (both will float in water).
If not, I suspect a plastic of some sort (but it doesn't look right).