I saw such cannons when visiting an Singapore Fire Service's exhibition some time ago. I was told that they are used for fighting fires at eg. oil refineries where oil are stored in hugh storage terminals of height about 60 to 80 metres. Such fires are fierce and extremely hot. Even firemen wearing silvery(in colour) fire retardant outfits and special masks to wear over their face (to protect their eyes looking at the fires), they cannot get close to the fire nor wise to do so in case of simultaneous explosions. Such ultra-high pressure water cannons are used. They can be mounted on boats to fight fires on hugh LGP carriers or tankers by drawing water from the sea.