That was dependent upon several factors, not last of which what type of boat was attacked. The British carried out full scale trials after the end of WWII on "S" and "T" class boats. They found that on the average (stress on "Average") a DC would caused no damage outside the 70-odd ft radius to a full size boat, mild to moderate damage within 70-20 feet and lethal damage inside 20 feet.
Conditions of target angle and depth of water affect the DC's performace, generally, closer to the surface, where the hull is less stressed, the DC must detonate closer aboard, as depth & pressure increase the distance at which damage is done increses due to incompressibilty of the surrounding water.
There is an article detailing the test results in Warship magazine (I'm not home and don't have it handy), if you like the full results write directly to my address above, 'll answer within 24 hrs.
Regards, Brian