"Smooth seas do not make skillful sailors" is a proverb you hear now and then. But maybe it is a little bit more complicated than that.
On the right side (in model above) you can see the "rough sea" that has at least two sides. One is of course the fact that it provides training, turning the sailor into a great sailor. But it is also dangerous and might cause the death of sailors. And maybe make other sailors abandon the job - becoming a farmer or tailor.
On the left side we can see that "smooth sea" creates romantic settings which can inspire to romantic meetings. And romantic meetings can result in children. These children grow up to become either sailors or non-sailors. And those who become sailors might get a chance - in rough sea - to become great sailors.
Being a great sailor increases the chance to return to harbor and have romantic meetings of course. Smooth sea also increase that chance, of course.
Another consideration is that romantic meeting might not always be good since the sailor might prefer romance over rough sea training. And that way missing out on training and fall back to become an ordinary sailor.
The yellow boxes are both needed, i.e. we need both rough sea and smooth sea to create great sailors. Not at the same time, and not with same purpose. But both still needed.
So, the answer to the question "What sea is best? Smooth or rough?" is always:
"Both smooth and rough."
Best answer to "or" is often "and".