If you look in Ratsey's book of sailmaking ca 1930, you will see a watersail described as a sail attached below a boom for the purpose of increasing the sail area of a fore and aft rigged sail when sailing before the wind [paraphrase].

The sail that is set on a yard (across the main axis of the vessel) in front of a ship is called a "spritsail." That is why the pointy spar on the front of a sailing ship is called a bowsprit. The sprit rig seen on fore and aft vessels except that they are both spars.

The spar that sits of top of the bowsprit, which most people mistakenly call a bowsprit is actually a staysail boom. It extends the spars forward to provide as long a base as possible for a sailplan. On vessels which do not set both spars, this is generically called a spike bowsprit.

I suspect that "a bone in her mouth" is just a variant idiom of "a bone in her teeth"for the bow wake on a vessel that is really tearing along.