Its name is a reference to walking the plank, a method of execution practiced on special occasion by pirates, mutineers, and other rogue seafarers.
What's harder than getting a zombie out of your backyard? Getting a wet zombie out of your pool!
Remember folks: "Save your brain, give your pool a drain."
While the hint to unlock this medal might not be very obvious, the requirements are almost the same as Pea in the Pool: you need to have a Captain Deadbeard stand on a pool. After doing that, the medal should unlock immediately.
A pool near the Out of Shape Human can be used to unlock this medal.