Tony -
During the earliest days of the first world war, before the invention of depth charges and ASDIC, a submarine could frustrate destroyers by simply submerging below ramming depth. There is a tale, however, of a submerged u-boat wondering why it was unable to evade a following destroyer. It turned out that the u-boat had snagged a bit of anti-submarine netting with an attached bouy; and the destroyer was following the bouy's erratic movements patiently waiting for the u-boat to surface within gun range. Fortunately for the u-boat, the destroyer lost sight of the bouy in the gloom of a rain squall.

Once the fundamental concept of a hydrostatically fuzed explosive charge had been developed, matters of appropriate weight of explosive and adjustable fuze settings evolved fairly rapidly. Early WW2 depth charges were leftover production from WW1. Developments unique to WW2 were fuzes with increased depth settings, weights and streamlining to increase the sinking rates of depth charges, and availability of more brisant explosives which increased the damage range of a given weight of explosive by increasing the intensity of the underwater shock wave.

I imagine you might find a few depth charge racks still attached to older ships donated to smaller navies after being declared surplus by the nations who built them. Depth charges are still fairly effective at discouraging low-tech anti-ship terrorist attempts by SCUBA personnel. =AL=