Would that be the "at the bottom of the ocean kind"?
Seriously, though, she might have boasted the largest armament in that war, but not the largest shipboard armament ever. The British deployed the HMS Furious with an 18" gun in 1917. The ship proved unsatisfactory for its intendend purpose, so a revamp removed the 18" gun. Three had been built, however, and were mounted on coastal monitors of the era.
Here's a pic of one of the monitors with an 18" gun. Notice how the damned thing is almost bigger than the tub its on.


There is also some indications that the US deployed coastal monitors at one point with 21" guns, but I'm having trouble finding concrete info on that one.
The Yamato does have an uncontested claim to the biggest AAAF gun ever mounted on a ship, evidently.