The firing depth also important as pressure increases by the increase in the depth which requires more energy
The pressure is equalized within torpedo tube before firing (i.e by introducing sea water to the tube at the depth being fired at).
There is thus no net effect from the sea pressure depth as the torpedo is essentially floating at fire condition. (it would be impossible to fire a torpedo at nearly any depth if this was not done).
What would matter w.r.t this would the water density (just like air density for aircraft/missiles) given the drag force created by it....but water is fairly incompressible (unlike air), thus the drag change is fairly negligible too (unlike air where it is advantageous to seek out less dense air for velocity increase).
The torpedo optimal firing envelope is thus dictated by things like what
@anmdt mention w.r.t negative surface effects (given eddies/turbulence with the air/water boundary) and also the torpedo construction (basic geometry + physics) being limited to a "crush depth". ...though this limits the firing platform (i.e the sub) w.r.t depth well before it does the torpedo....given their relative thin wall volume rigidities.
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