Sure, Leonardo da Vinci created some great art or whatever, and some cool inventions that make life easier. We also found out he was a weapons developer and he designed some badass military devices.
Then we took a closer look at those designs and realized Leonardo's a lot less artist and a whole lot more Darth Vader.
"And if it should happen that the engagement was at sea, I have plans for constructing many engines most suitable either for attack or defense, and ships which can resist the fire of all the heaviest cannon, and powder and smoke."
- Leonardo da Vinci
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?
As for the scuba gear, it was specifically designed for "sneak attacks on enemy ships from underwater." Just the loss of a few enemy ships while in dock would have been enough for every navy on the planet to fear you.
Like this, only with more explosions.
The gear itself was incredibly complex and even included a pouch for the SEALs to piss in. The plus-side: Much like modern SEAL units, urine keeps you warm in cold waters. The downside: None. The competition: None. The possibilities: Endless. With Venice stormed by your SEAL team and Genoa scared into submission, you would be the undisputed ruler of the Mediterranean.