During the Cold War, East and West both trained combat divers to sneak into enemy harbors, spy on warships and stick mines on them. In 1956, when a Soviet warship visited Britain, a British diver working for MI6 swam under it to spy. He was never seen alive again.
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Stopping divers like him was the hard part. Soviet guard divers went down with a knife and an AK packed in a waterproof case, useless until they came back up. Ordinary guns barely work underwater anyway: a .50 cal bullet that can wreck a truck engine breaks apart in less than a meter of water.
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So Soviet engineers swapped the bullet for a steel dart about as long as a pen. Its tip punches a bubble into the water, and the dart flies inside it. In 1971 their divers got the SPP-1, a pistol with four barrels and one dart in each. Four years later came the APS, an underwater assault rifle with 26 darts in the magazine.
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Underwater, the APS could hit a diver up to about 30m away. On land it fired too, but wore out after about 180 rounds instead of 2,000. So Soviet divers simply carried two rifles, an AK for land and the APS for underwater.

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Meanwhile in West Germany, Heckler & Koch built the P11 for German combat divers: five sealed barrels, fired electrically by a battery in the grip. After five rounds, the diver swaps in a fresh barrel pack. The empty one gets shipped back to the factory, the only place that can reload it. US Navy SEALs carried it too, even though for years H&K wouldn’t admit the gun existed.
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As far as anyone knows, East and West never fought a single underwater gunfight. In 1989, US President George Bush and Soviet leader Gorbachev met on a ship off Malta, at the summit often called the end of the Cold War. Soviet divers, reportedly carrying the APS, patrolled the water below, and so did American navy divers.

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Both sides had spent decades building guns to fight each other underwater. In the end, their divers guarded the same summit.
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Photo credits: Slide 1: Vitaly V. Kuzmin (top); Soviet State Archives / Press Service (bottom). Slides showing Soviet divers: Mil.ru