Here's a pic out of many others that I've kept from my Military History class at the University of Alberta in Canada.
It's that of an underwater SPP-1 4.5 mm pistol. The weapon is fitted with a four-round magazine. It fires a drag-stabilised dart with sufficient power to deliver a lethal effect at 50m range in air or up to 1m range underwater, depending on the depth of the water.
It is used by a Russian Special Forces combat diver who would also be equipped with a close-circuit aqualung.