Stress

A writer gets stressed out by looming deadlines. A doctor stresses over figuring out a problem with the body and fixing it. A sailor gets stressed out over the threat of their ship being capsized by a looming storm. It's all relative, right?

 
Remember: a pirate ship looks just like a regular ship, except...more pirate-y. (Source)

Maybe, but something about the constant possibility of death via the whims of Mother Nature just seems a tad more intense than your average job. Aside from the threat of losing your life, there's also the threat of losing your cargo, getting lost at sea, or running into pirates.

Let's be honest, though. Anyone looking to become a sailor isn't going to shy away from some high-stakes stress. Life at sea is for those who love adventure and are ready and willing to put their lives on the line for a career that gets their hearts pumping and their sea legs quaking. If that means a few extra gray hairs, so be it.