Crush Depth rant

[Hooston]

Rant ye not! You should be able to get to at least 200 metres in an undamaged boat.
Most likely you hit the seabed. The SHIII seabed is full of very pointy rocks, or possibly very ill natured lobsters. If you touch it very gently at slow speed you will get away with superficial damage, either not shown at all or repairable in a few minutes. There is a mod that makes you harder to detect on sonar if you are near the seabed but if this is not installed it's a good idea to stay away. Use the depth sounder when it is safe to do so and keep a track of what the local sea depth is. The chart is not very accurate. As a last resort check using the external view.

GWX does have some historically accurate deep minefields in the Irish Sea, but i think they will be about 20 metres down.

GWX aircraft are also armed with depth charges that can go quite deep, although in real life the allies realised fairly soon that a simple single shallow depth setting was more effective. A problem with GWX is that if you give away your presence the escorts and aircraft head straight for your position with "laser precision" and attack. if you have the external views enabled you should usually get a pop-up window if an aircraft attacks.
Last possibility is a late war acoustic homing torpedo launched from an aircraft late war - if you have the HSie/Steibler mod and the right options.