i think it is true that the deeper you can go the more chance you have to survive a depth charge attack. all historical info i have read confirm this. from a physical and tactical point of view going deep reduces the lethal range of a depth charge becuase the big water pressure reduces the shock wave that is dropped against the hull of the boat. of course- the hull itself is in great tension by the water pressure. but that seems to be more than equalized by the reduced shock wave of the bomb in deep water. the second argument ist that especially in the first years of the war german uboats were able to dive much more depper that the allied forces believed and so they dropped their charges at relative shallow depths with no effect to a deep dived boat. third is that at deep dive you have more time to outmanouver the charge in relation between the point it is hit and it has sunk to a determined depth and explodes.

of course- a direct or nearly direct hit with a depth charge is a sure kill at whatever depth the boat is, no question.