you sure you weren't damaged? if your hull integrety is less then 100% it will decrease the limit you have as far as crush depth.....the default sh3 has been modeled that if you have say 10-20% hull integrety if you dive 5-20 meters your hull will collapse....as opposed to its normal crush expectancy. you can also change and modify its values using time traveler's crush depth tool:
http://www.delraydepot.com/tt/sh3.htm#crushdepth It will allow you to adjust the depths of each sub to realistic values.....and randomize the %'s to give you that uncertainty feeling....as the true crush depth is not really known for each uboat.....the default game has all subs crush depths relatively the same...and timetraveler's tool reallyh helps correct this. He even includes a guild at bottom of tool on average real world figures taken from different sources.