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.