OTL: Around 1900AD Davis designed an oxygen rebreather breathing set to escape from sunk submarines. Afterwards industrial oxygen rebreather breathing sets were invented, but nobody swam underwater with one, until in Italy around 1935 sport spearfishers started using them. That is why Italy is the only nation that started WW 2 with frogmen.
ATL: 1910: Hamburg (Germany): Inside a diesel tug's engine room a mechanic is working on something while its engine is running. He has a dizzy turn and loses his grip and his footing and falls heavily on its exhaust pipe, which is weakened by rust. It breaks, filling the engine room with fumes. Johann Schmidt, docks mechanic. puts an industrial breathing set on and goes to turn the tug's engine off and attempt rescue. On a gangway he slips on spilt grease and falls into the dock into 20 feet of water, weighed down by a bag of tools. He is very cold but the breathing set keeps him alive. He sheds the bag, and this leaves him about neutrally bouyant. He must stay submerged due to danger from surface craft and their lethal spinning propellers blundering about like elephants looking for him. He breast-stroke swims underwater to the dock wall and climbs out up a boat-ladder. This gives him or his foreman an idea for a way to dive much lighter and more agile than hardhat gear, and undetected. The frogman's drysuit is soon invented. Someone adapts a submarine torpedo as a man-rider. Germany starts WW 1 with frogmen.
Or translocate this to some other nation.
ATL: 1910: Hamburg (Germany): Inside a diesel tug's engine room a mechanic is working on something while its engine is running. He has a dizzy turn and loses his grip and his footing and falls heavily on its exhaust pipe, which is weakened by rust. It breaks, filling the engine room with fumes. Johann Schmidt, docks mechanic. puts an industrial breathing set on and goes to turn the tug's engine off and attempt rescue. On a gangway he slips on spilt grease and falls into the dock into 20 feet of water, weighed down by a bag of tools. He is very cold but the breathing set keeps him alive. He sheds the bag, and this leaves him about neutrally bouyant. He must stay submerged due to danger from surface craft and their lethal spinning propellers blundering about like elephants looking for him. He breast-stroke swims underwater to the dock wall and climbs out up a boat-ladder. This gives him or his foreman an idea for a way to dive much lighter and more agile than hardhat gear, and undetected. The frogman's drysuit is soon invented. Someone adapts a submarine torpedo as a man-rider. Germany starts WW 1 with frogmen.
Or translocate this to some other nation.