Hi!
From what I've read, the flooding that occurs in your u-boat is generally due to the failure of gaskets, flanges, etc., around existing openings in the pressure hull rather than a breach in the pressure hull. The existing openings are for things like the plumbing for moving water in and out of various ballast and trim tanks, exhaust vents for the diesels, periscopes, etc. This kind of damage will slowly kill your U-boat if you can't repair the damage and pump the water out, whereas a breach in the pressure hull itself will likely sink your boat in short order.
The "water hammer" effect is caused by the expanding gas bubble formed by the exploding depth charge. Water cannot be compressed, and so it transmits the power of the explosion for a much greater distance than would be the case in air. This overpressure is what tends to cause slow leakage, things knocked about, etc.
If the depth charge is very close, and the gas bubble from the explosion actually touches the U-boat then the U-boat will also feel the "thunder" effect as the gas bubble collapses under the pressure of the water above it and that sudden pressure slams into the U-boat's hull. At shallow depth, the water pressure is less resulting in a bigger bubble but less pressure; in deep water, the water pressure is much greater, leading to a smaller bubble that collapses with much greater force.
This "thunder effect" is what allows a Hedgehog to kill a U-boat with only a very small explosive charge at any depth- the Hedgehog only explodes if it is in direct contact with the U-boat, and if it's really deep then the pressure hull takes a wallop from water at a pressure of many thousands of pounds per square inch - not to mention that a leak under that pressure is going to look and feel like a super-pressurized fire hydrant.
Pablo
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