Sometimes when TNT detonates in my world it destroys blocks even if it is in water. I found this out while building my new TNT cannon.
I no longer have a TNT cannon.
This might be caused by the new water physics in 1.13. Also, the TNT that exploded and destroyed terrain was not fully submerged in the water, some parts of the entity's hitbox were outside of water.
Can you provide a screenshot of how you setted up the cannon? And a video of the tnt exploding inside of the water