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Everyone knows about da Vinci's designs for flying machines and tanks, but his dabbling in submarines is often overlooked. He designed a submersible craft and a diving suit which he presented to the Republic of Venice (naturally, it was turned down). Then after a brief interest in the 16th century, it seems that with the exception of the American Revolutionary War the ideas fell by the wayside until the 19th century, and even then, the idea was met with polite indifference until the 1890s. http://didyouknow.org/submarine/
So what I'm wondering is what is the earliest plausible date we can establish bubbleheads as part of the armed forces. Obviously, this would require a sustained interest in the idea of underwater warfare rather than it moving by fits and starts as OTL. Also, who would be most likely to be the first country (Venice? England? The United Provinces? France? Someone else?) to takes this idea seriously?