Good story, but I have some notes.
The beginning is kinda like a fairy tale, with a summary of the MC’s life, and the rest of the story is a twisted fairy tale of sorts. An orphan child protagonist, a witch, talking to animals...the setting is very "fairy-taly", but with some twists.
The fairy tale narration has its cons though. The one that caught my eye was the weak descriptions. Think about the Sea Witch for instance: “[she] had no bones, and no skin, but hung together within herself as a jellyfish might.” The image could’ve been better conveyed here.
I had an issue with the MC, Castermay. She's too stoic. On the one hand, her character deviates from the norm that we see in fairy tales; on the other, she just doesn’t feel human. That means she’s not belieavable. Maybe the author has tried too hard to write a deviation of a fairy tale protagonist.
I had to google "gannet", and I found out that, in British English, that's slang for greedy person.
Finally, this reminded me of Ponyo (2008) a bit. Even the theme of "care" is present. Too bad that's one of the wrost Ghibli films ever.