I'm playing Kindle Roulette with Vysed, and this is the book that came up

Salt Wars (Xavier Hollands stories) - Derek McMillan, Angela McMillan

"Then there was the very important coded message.  Of course it was in gobbledygook.  Fortunately Xavier had had an enigma machine as a toy when he was eight so it was second nature for him to configure it according to the "idiot's guide" instructions he received.

 

he's a much smarter kid than I was.  Of course, they all seem to be precocious in tech these days.  And who has an enigma machine for a toy?

 

anyway, interesting so far.  (78% done)

 

Oh, update at 80%. They are on a submarine.

 

"What happens in the torpedo room, stays in the torpedo room." 

 

An overused phrase, but it gets the point across succinctly, I guess.