How is that Hydrogen stored?
The engine has never been the issue, just like electric motors have never been the issue holding back EV.
it would be cool if you could extract it from water
Probably in a metal hydride. Similar to a NiMH battrey:
https://www.fuelcellstore.com/metal-hydrides
And how is that hydrogen manufactured and at what cost? This is a big fat nothing burger.
Explosively.
Stored? How is the Hydrogen produced and at what cost? This engine may rival a diesel engines efficiency, but I seririously doubt it will be able to match diesels cost.