JR the site seems to have lost your post, so I will respond from memory.
You see no problem with a mile deep of supercritical water being emitted from earth at mach 151. That is 1 mile deep, 60 miles under the surface of the whole earth.
All this water equal to 1 mile of earth's surface all blowing out of crevasses at mach 151.
And you ask me to explain how this would destroy all life on earth.
If your hydroplate buddy had a real science degree, not one from the university of Mom and Pop, he would have known and explained why this would instantly kill almost all but bacteria on earth.
Here is the size of what scientists call doomsday or killer meteors...
"
In what could be a major scientific puzzle, the team's new size estimate for the dino-killing meteorite is a mere 2.5 to 3.7 miles (4 to 6 kilometers) across.
The most recent computer models predicted a size of 9 to 12 miles (15 to 19 kilometers) across."
https://news.nationalgeographic.com/news/2008/04/080410-sea-meteorites.html
So a lump of rock a mere 3 to 12 miles across was able to wipe out the dinosaurs so that none survived.
And these rocks travel at about mach 70.
Yet this pinprick sized rock can cause a mass extinction because of its velocity.
What you and your hydroplate buddy propose is a reverse of a meteor striking earth. Same energy principle.
Except you and your buddy have an earth sized sphere 1 mile deep of water travelling at mach 151 bursting from earths surface.
I don't want to do the maths but this volume of water is millions of times bigger than the mere 12 mile meteor, and twice the speed. The energies are incomprehensibly large. That means it would do billions of times more damage to earth than the dino-killing meteor. No wooden boat, covered with bitumen could survive this. No cockroach could survive this. If you and you buddy cannot comprehend this amount of energy, well, at least I tried.
You also asked why the mach 5 water jet does not disperse. It does! That's why they put the steel close to the nozzle.
And once water leaves extreme pressure and heat it instantly is no longer supercritical. So all the magical properties disappear.