Simple really. an army uses maps, and the wet bits are in blue which the army knows it needs boats for, therefore they try to avoid like the plague. All Armies are the same, and so a gentlemens agreement is that trenches should stop one mile short of the beach otherwise the soldiers might have gone swimming at a cruicial moment in the battle, and that wouldnt be cricket! /forums/images/graemlins/laugh.gif
My elder son is at the Biggles-reading age. In one story our hero sets eyes on the very end of the trench system, with the barbed wire, mines and cheveaux de frise extending into the Channel. I assume WE Johns was describing something that he actually saw.
The trenches "met" the sea at Nieuwpoort in Belgium.
The advancing Germans were stopped when a lock keeper in Nieuwpoort opened all the locks on the river Yser and all the lowlands behind them flooded.
So there was no need to dig trenches all the way to the sea.