I would think a hose would let go before any serious pressure could build. I've blown a hose off my radiator with just my garden hose that deadheads at about 60psi at the faucet. I've done just water on the street when I had a serious leak I didnt have time to fix.....bad idea though. Lots of corrosion.
The radiator cap should blow into the coolant tank.
The problem is that crud and rust can build up and cause a blockage.
That lets pressure build against that blockage.
Any void or right angle becomes a steam pocket.
When that blockage fails from the pressure of that steam.
That super heated air water mixture will immediatly find its way to equalibrium.
Through the path of least resistance.
If you are lucky thats through the head gasket.
If youve been running pure distilled water and it ate your aluminum radiator it might just explode in your face and burn the **** out of you.
Coolant isnt some kind of a scam.
Spend the 15 bucks