riddle - This is for Duck

As promised I have crafted a homonym riddle just for @Duck. With the recent spike in homophone riddles, I figured it was time. I finally figured out what word to use, so can you tell me which word it is?

Depart quickly, turn back now; this little riddle is going to be foul.

Take a plunge, into the fire; like learning to walk on top of a wire.

Don’t cower, nor cringe; there’s no need to fear; solving this riddle, won’t take a year.

Tipping already? Biting your nails? The next clue is, I am used within sails.

Help is here, an unsung hero; also known as batsman’s zero.

If that’s not enough, then I’ll get you out; my vehicle floats, there’s no need to pout.

One last clue, before I must leave; defined as another, a mollusk I be.

What am I?

Answer

Are you

Duck?!?

Depart quickly, turn back now; this little riddle is going to be foul.

Duck-out, and (foul), are types of birds (thanks Jafe :)

Take a plunge, into the fire; like learning to walk on top of a wire.

Plunge like dunk and dip is synonymous with duck.

Don’t cower, nor cringe; there’s no need to fear; solving this riddle, won’t take a year.

Cower is another synonym like duck down or cower.

Tipping already? Biting your nails? The next clue is, I am used within sails.

Doetch is Dutch for cloth, which makes up sails. Doetch later turns to Duck in the English language. (From R.D and Duck thanks :D)

Help is here, an unsung hero; also known as batsman’s zero.

I typed in "Batsman's Zero" and duck was the answer.

If that’s not enough, then I’ll get you out; my vehicle floats, there’s no need to pout.

Ducks float, also pouting is close to a duck face (Thanks fogmeister)

One last clue, before I must leave; defined as another, a mollusk I be.

And also there is a mollusk named the Pacific Geoduck(From R.D and Duck thanks :D)

But,

This puzzle IS for Duck.