The first third of the book moves more slowly than I remembered; there's a bit of set up as the Swallows meet Jim Brading and arrangements are made to do some local sailing on his boat, Goblin. It's slightly horrific to realise that Jim, who seems so grown-up to the Walker children, and who smokes a pipe (!!) is presumably only 18 or 19 and just about to start university... only a few years older than John. The long set-up also serves to reinforce Jim's fear of losing his boat to unscrupulous 'salvage' operators, which is why the children are later so reluctant to ask for any kind of help.
However, once the fog sets in, Jim goes missing and the Swallows find themselves drifting helplessly out into the North Sea, the story goes at a cracking pace. The middle third is the most harrowing, as gripping and suspenseful as any book I've ever read. John and Susan usually appear in the series as the sensible, capable elders, quasi-parental figures who can handle anything. But here they are really tested -- John is taken out of his comfort zone, he makes mistakes, he is sailing a boat larger than any he's been in charge of before, and having to manage pretty much single-handed in wild weather. But he rises to the challenge, thinks things through and mostly makes the right call, after his first dreadful error. Meanwhile dependable Susan is rendered utterly helpless by seasickness ("ough...ough...ough..." -- it's horrible to read, poor Susan, I know how she feels) and it's she who insists they go on rather than turning back, because she simply can't bear it. For the first time we see John and Susan really rattled and arguing as they are driven on and on through the wild, stormy night.
Mrs Walker's premonition that something might have gone wrong might be the second instance of the paranormal in the series? But the final section of the book provides much needed relief and a fun cruise to the finish line as Goblin arrvies safely in Holland and the children's father turns up to take them home.
'You'll be a seaman yet, my son.'
And John, for one dreadful moment, felt that something was going wrong with his eyes...
Yes, it's called emotion, John, deal with it. We Didn't Mean to Go to Sea is an absolutely terrific read.