Three words: Containment Foam Shells.
Hey, 'sup, how's it going? I wanna show you something regard Naval Artillery, so just bear with me okay? Particularly since I have to reread somethings...
Okay, so below are pictures of Iowa and Wisconsin firing full broadside, and I need you to pay attention to the water:
The disturbances you see in the water are caused by the shock waves from the guns firing. In the top picture, the shock wave causes a disturbance with a diameter a little more than half the length of the ship, and furthermore, there is no one on deck, because they would be killed by the shock wave. Just keep this in mind, I'll get back to it later.
The Yamato's guns were designed to fire shells of 1,360kg and 1,460kg in weight at distances of up to 42km at maximum elevation, with a muzzle velocity of 780m/s. While the Bismarck's own guns fired 800kg shells at distances of up to 35.5km with a muzzle velocity of 820m/s.
Now, where am I going with this? The Containment Shells you are saying that can be fired from either ship to reduce their lethality will need the following:
1) To be designed to fit either gun so that they can be fired in the first place (46cm for the Yamato, 38cm for the Bismarck)
2) To be fired using the correct amount of charge so that they can clear the gun barrel, the ship's immediate area, and reach the target
Remember that point I made about the shock waves of the Iowa's Mark 7s being lethal? A shock wave is just air, literally just...air.
The speeds that the Containment Foam fired from the Yamato or Bismarck will reach WILL make them lethal. Sssssoooooo, my suggestion? Shelf that idea, it would only lead to increased collateral damage and needless loss of life, on top attracting more violent Parahumans who would want to try to take on a battleship just so that they could say that they won.