Seems like you had to learn the hard way! Sorry about that..
I'm going to be blunt and to the point here so that it helps others!
NEVER, NEVER, NEVER, EVER CONNECT PRESSURIZED WATER OF ANY TYPE TO YOUR RAW WATER INTAKE ON THE ENGINE!!!
I tried to explain just that to one of those "new boat
owners", who "knew it all", two years ago one Saturday at the yard, and he proceeded to total his engine within fifteen minutes of me trying to explain the reason for not hooking it up that way! Over the years I've seen or known of at least five engines destroyed in this fashion. Dave you got very lucky that your engine was NOT damaged.. The guy at the boat
yard actually hydro-locked his and destroyed connecting rods and more. Either way my boat yard made about 12k on that guy... I offered him my 5 gal bucket but..............vroooom vroooom boom...
If you don't fill the muffler and back feed the engine that way the pressurized water will, in many if not most cases, blow by the seals
in the water pump thus forcing water directly into the engine! This problem has NOTHING to do with your impeller and everything to do with how NOT to flush an engine or run an engine on the hard
THE RAW WATER PUMP IS A PUMP AND IS NOT DESIGNED TO BE FORCE FED!!!
Sorry for the abrupt candor but folks need to understand this is NOT good for the engine and can cause catastrophic damage..