Yeah, right there is everything you need to know about how Leonardo da Vinci approached military design. That's a fucking ancient submarine holding a gigantic cartoon knife. And Leonardo is known for the freaking Mona Lisa? Look at that thing!
But what good is a submersible ship-cleaver without a SEAL team to rig charges?
That's right. Leonardo designed his own SEAL team.
Why it Would Have Worked:
The Crusades turned city-states like Venice and Genoa into superpowers thanks to the usefulness of their mighty navies. If anybody was going to stand a chance against these two powers, they needed men like Leonardo to sink their fleets swiftly and silently. Underwater warfare would have turned every drop of water in the Mediterranean into a war zone that no ship would be safe in unless they had pledged allegiance to your kingdom. Your fleet would have been as feared as Poseidon himself.
Seriously, could this possibly be any more pimp?