Although there are many possible interpretations to this sonnet the reason why Castalia gives this to Kane is quite clear: because love is a lighthouse and a sailor might understand the metaphor.
The storm came in and almost swept her away, and of all the reactions she might have had - because she almost died - she is giving Kane the bravest: the world is dangerous and scary but it does not matter because you are in it. That's the interpretation she gives to the poem.
Did you remember when she scolded Kane about not understanding things unless they are written in a book? There! She found a book he can understand. This puts his mind at peace.
Just imagine what Kane must have gone through while Castalia was sick, in bed. He kept telling her that he was not going anywhere without her, that's a scary thought, if you stop to think about it.
He probably thinks, now, that Castalia is the lighthouse too and therefore understand that what they have really is a marriage of true minds.
The fact that she found the book all by herself, going through God knows how many in his collection, it's something that shows dedication, patience and the will to continue with the life she has, no matter what the cost.
Because of what she just told him: comes what may... I'll take it all.
She already almost died and no matter how much he tried to protect her, it still did not work. So he can only trust her to take the truth in her hands and decides to share with her a heavy burden.
Now we know that Kane's mask is meant to protect others.
Think about it, if you don't know about it, you cannot talk about it. Of corse the protection is meant mainly for Josh. But in a way it's also meant to take attentions away from the ones around him, by making himself and easy target.
Imagine if you knew, what reaction would you have when you see Josh, you would think: is this him or his brother?
Or if they stand next to each others, people would look at them all weird.
Also there are less chances the information might leak while somebody is drunk.
So why not just cut their hair differently? I would not be enough. Assassins don't go for details, when it comes to take lives, they take it then... ops, I made a mistake, and go after the right one.
The mask is not only something to differentiate the brothers, it's something to grab attention.
Have you ever read the explanation Frank Miller gave to batman logo's being so bright and smacked in the middle of his chest? It's because it attracts attention to that area and villains would shoot him there (where he has a bulletproof vest) instead of shooting his leg, shoulder, arm. It's psychological.
So Kane thought that the mask would create the mystery, attract attention on him, it's all part of the fame and curiosities that would point at him as a target and not at his brother or his men.
An assassin is told: the pirate Balthasar, you can't miss him, he wears a mask.
The danger is focused on Kane himself, everybody else is safe.
So sorry for those who thought there would be a scar or something else, truly Kane would not be the type to cover a scar, cocky as he is, he would wear it with pride.
Usually a mask is there to hide someone's identity, like for Zorro.
This case is the opposite: the mask is there to truly identify him as The Pirate Balthasar, the one and only.
As for the tan line... you already forgot, mumble mumble, Castalia once said that the reason why Kane likes to go on the crow's nest to read is because he can take the mask off and sun bate! XDD (he is still a bit vain, after all).
The skin reaches a level of saturation after which it cannot get any darker.
Now if I were to draw all the tiny tan lines these guys should have around their eyes, because being in the sun they squint, or around their necks (because they all have red necks), there would be no way for me to come out alive from making this comic.
As I once said, I took the decision to simplify many things: the rigging, the saltiness and the wind in the air, the level of noise, the sailing language. It's not like I started super detailed and then simplified it, I tried to stay coherent since chapter one.
Besides, if you think about it, Disney's Pocahontas did not even have fingernails but everybody just assumed they were there.