In 1942 the British had a 500ft (i.e. 150m) setting so bearing in mind the height of your boat above the keel and the inaccuracy of the fuses (and the possibility of soap!) that sounds entirely realistic. i'm not sure if the fast sinking charges with extra weights gave different detonation depths to those marked on the fuses.
Guessing your depth would be down to the experience of the warship crew, and that is captured in the game.
There's no telling what the game settings are in any particular mod soup. To be honest that's a good thing as it forces you to default to real tactics rather than game exploits. Whilst it adds greatly to the fun it's best to not use the external views too much as it tends to make problems with the simulation too obvious. If you ever get a pack attack on a convoy you can see many crazy things via the external view. The exception is if you sink the whale factory ship in GWX - external view is then essential!
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