Did Pixel mean that the sub could have been boarded while the Amazon was stuck on top? How did the real story turn out? How could you board a sub if you were a boat stuck on top?

very well described

Why did the U-Boat captain not surrender after disposing of his secret codes and stuff? With no guns and the sub disabled, the Amazon would have been obliged to board and take on prisoners

Lovely. Tom and John on a Destroyer together. What were the chances of that? I won't complain if you write another chapter, but, that said, it works really well as it is. Reading anything to do with 'Swallows and Amazons' always makes me nostalgic; I knew more or less how to sail before I learnt to sail from reading those books (if that makes sense). Anyway, lovely.