You cannot hide.

[Surpmil]

It was an awfully expensive way to sink u-boats. I wonder if any attempt was made to plant timed explosives in them using French dockyard workers.

Considering just the economic and human costs, frogmen with limpet mines might have been a better bet, if adequate time fuses could have been provided. Something activated by sea water erosion or corrosion perhaps.

A boat that sinks while out on patrol left no evidence of the cause in those days.

Torpedoes would have been an ideal target if their warheads could have been detonated somehow, say when their firing sequence began...