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I wonder if you could try cutting down run-on sentences? For example, the first paragraph could read:
"Amid the swooping and crying of gulls, Cathy, still groggy, had joined her father and Stevie at the starboard rail. Stevie was standing atop an upended bucket, his father’s arms around him so they could both grasp the same rod; an arrangement Cathy remembered well from her own childhood. Where father's and son's fishing line met the brine, a cork float bobbed and dipped, as it trailed the Ormer on a calm sea.”
You might also want to reduce the number of sentences that begin with ‘and’.
I know when you come up with descriptions it’s hard to decide which to use, but you should probably only use one comparison. For example, in this sentence:
“She looked to port way off to the south-west and glimpsed the red and white rings of Casquets Lighthouse on its long low ridge of barren sandstone rising from the depths like an immense row of jagged teeth or a sea monster's jagged spine.”
Use either the jagged teeth simile or the jagged spine, but not both together (personally I prefer the jagged spine of the sea monster.)
Another possible paragraph alteration:
“With her hands on the wheel, Cathy glanced over her shoulder and noticed the moon had risen early, like the lopsided smile of the Cheshire cat as it reflected the sun’s strong rays. There was her M'mée behind her, fully decked out in black oilskin trousers and coat with a sou'wester on her head and sea-boots to her knees.
She was resting her elbows on the bulwarks for a long stare through les longues-veues, ‘the long-views’, as she called Dad's heavy field-glasses. The Ormer had brought the Johnsons and all their cargo beyond Alderney, but on this very clear day, Marie might see the northernmost point of Jersey some thirty land miles away.”
I don’t think mentioning the railing of the deckhouse necessarily adds to the narrative. As a side note, why is the mother in full storm gear if it’s not raining?
Anyway, you have some beautiful descriptions in there. :)

I think Cathy will have to take the Authoress aside and tell you to kill Robert off in the war, which would solve everyone's problems. :)

I think it's really interesting how the whole of the British Isles have regional black dog legends...I don't know if it extends to mainland Europe. I think it might be particular to the UK. Also, Cathy's dream about the ormer police was really funny. :D

Poor Robert, why doesn't she like him? :)

You should probably italicize names of songs or put them in quotation marks so they can be differentiated.

Can we get a cow? Dexters are really small.

"Thou shalt not cover." I like that. :D

I feel like all my effusive comments about cows have come back to haunt me. :P How common is red hair in that neck of the woods?

I feel bad for Cathy, but I understand her mother wanting to go back.

You should have the mother tell some cheerful stories with happy endings at some point. Maybe some French fairytales?

The adventure continues!

A few thoughts:

I believe the Japanese invaded Manchuria in 1931.

Is it plausible that a fishing family could afford to send their child to a boarding school? If so, wouldn't he have taken on the accent and vocabulary of the other boys in order to avoid being bullied?

Does the father have a good reason not to take the cat? is it a superstition? Cats are ubiquitous on boats.

The address on the letter is pretty funny. Was that based on anything in real life?

This looks like my kind of story. Followed and will be reading!

Looks like a good start to an adventure! :D