If you run SH3Cmdr, let it do it for you.
Simply open "SH3 options.cfg" and look for the [CRUSH DEPTH] block. Change the number at the end of each line (or just the line relevant to the u-boat you want to test with) to any 4 byte single number (ie between 1.5 x 10^–45 and 3.4 x 10^38).
You can also change Lower=0.9 to Lower=1.0 to force SH3Cmdr *not* to randomise the depth at all.
Nb: No blaming SH3Cmdr if the value you set causes CTD's or other instabilities.