Do you agree that a smooth sea has never made a skilled sailor - why yes/no?

in the quotes simplicity, we could argue that skilled sailor is not limited to stormy weathers.

meaning behind the quote is to appriciate the hardships we have. we become great due to the burdens we carry sometimes. for example two bike riders come to hill. rider A never went up hills before, but rider B has. rider B is assumed to have easier time going up the hill.

I agree with the quote to a limit. how we approach a storm can be different for each person.

Taken literally, yes, because presumably a skilled sailor is one who can sail rough seas.

In order to improve at anything, the difficulty of the task must be matched to your skill level. When playing piano for example, you learn a piece slowly at first, then gradually increase speed to get the hang of it. When playing chess, you'll quickly plateau if all you ever do is play the computer on level 1.

A martial artist might train for 10 years before getting in the ring, but if he never trained sparring, he'll still be unprepared (you see this a lot in "martial arts master gets destroyed" type videos).

However, I think that in practice the quote is often used similarly to the saying, "Hard times create strong men. Strong men create good times. Good times create weak men. Weak men create hard times." In that case, if psychological coping, rising to the challenge, etc., are skills, then like I said above, the difficulty of the task must be matched to your current skill level. If someone is bullied in school, they don't automatically become a strong person - more often than not they experience psychological trauma that affects them for life.