Without a doubt, with World War two not occurring, hydroplanes wouldn't have been invented. The first and foremost requirement of the early hydros is that they be liquid cooled. Only the first generations of fighter planes used liquid cooling. The P-40 Warhawk was one of the first.
The Warhawk was primarily used for ground attack but was a formidable dog fighter as well. The Canadians primarily flew them during the war. Powered by Allison 12 cylinder engines, they cranked jp about 1100 horsepower. The more famous of these planes though were the ones flew in the Burma-China theater. They were referred to by the Chinese as Flying Tigers. The Japanese having a different perspective, called them devil planes and other names unfit for print.
Though the Japanese had superior planes, the Flying Tigers had Superior training. All of the pilots were given discharges from the US Navy and Army Air forces for the purpose of enlisting in the Chinese Air Force. These were top of the line pilots, instructors and other experienced people including of course enlisted mechanics, armorers and such.
The U.S Army Air Force chose te slightly faster and more maneuverable P-51 Mustang. The P-51 early models used the V12 Allison as well. They were some of the first supercharged engines ever used. Later P-51's switched to Packard built Rolls-Merlin engines. These were british planes built in the USA.
The British Spitfire, also using Rolls-Merlins were the follow on fighter that stole most of the glory from the Hurricane, which was the plane that won the Battle of Britain. Early in the war, much like the United States, the UK was not prepared to go to war. The brits dusted off some plans for a low wing fighter, added a better engine and took to the sky driving back the German ME-109's and others including the Heinkel bomber that indiscriminately bombed London. That prompted the British to return the favor, obliterating German cites.
The end of the war in 1945 created a huge surplus of these planes. Canada sold over 100 of them at $50 per plane. With all of these high performance liquid cooled engines available, there were a few racing enthusiasts with a need for speed. Thus the birth of modern era hydroplane racing. More to follow in a series following the history of the Unlimited Hydros, or as they were known, Thunder boats.