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.