At the end of WW2, the Japanese had developed, but not yet deployed, suicide airplanes with 2,900 Kg shaped charges built into them. If they were deployed, and managed to get close enough, it's possible it would have blown right through the 12" armor of an Iowa class battleship.
It was Mitsubishi Ki0167[2], according to Ryan Szimanski the curator of the Battleship New Jersey, in this video[1].
This marked the effective end of the concept of armor thick enough that it couldn't be breached, and the Battleship.
ok but that's 12" armour. But doesn't necessarily mean we went to the other extreme, i.e. that a modern military ship has the same hull than a trawler. A quick google search suggests the thickness of an ice breaker hull seems to be about 2 inches.
It was Mitsubishi Ki0167[2], according to Ryan Szimanski the curator of the Battleship New Jersey, in this video[1].
This marked the effective end of the concept of armor thick enough that it couldn't be breached, and the Battleship.
[1] https://www.youtube.com/watch?v=9KlFxbF8hrs
[2] https://www.daveswarbirds.com/Nippon/aircraft/Ki-167.htm