I PRESUME that with a jet boat, no flushing at all is necessary
because not much of anything touches the water. ????

Actually, the thing is always in the water since water runs through it. Make
sure all the zincs are kept up with.

As to how I got into the jet boat area, I needed a shallow draft
because I live on St. George Sound in FLorida and the water gets VERY
shallow, more draft than a foot would not allow me to moor a boat
within 1000 feet of my house.

Operating with less than 12" between the pump inlet and the bottom will lead
to early pump erosion.

My only problem has been wondering why ALL boats are not
jet boats - because it seems that the advantage of not having a
propeller to get wrecked and needing to be maintained would be a
no-brainer. So, I KNEW there were some disadvantages that I was not
aware of - such as stability, longevity, maintainability, size
limitations, etc.

Efficiency is a major factor. You have an efficiency loss with a pump. Just
because you don't have a prop to destry doesn't mean you don't have an
impeller to destroy. You catch a piece of line on a prop, and most likely
you just have to cut it off, you suck it into the pump and it may cost you a
couple thousand. My favorite choice of drive for shallow water is a surface
piercing prop like an Arneson. Every drive system has its advantages and
disadvantages (except I/O, I can't figure an advantage to that one), you
just have to figure which works best for your situation and try to work
within the systems limitations.