You won't wreck a good impeller by cranking the engine or even starting it up briefly with no water to the engine.
My usual routine if I have to start it out of the water (at home), since I just put a garden hose in the r/w line, is to start the boat with the hose ready to go, then go turn on the hose. It runs for 10-15sec dry, no problem.
I had an unfortunate incident with a defective Perko flush pro first time out a couple years ago. Boat idled for proabably a couple minutes before I realized what was going on. The engine compartment got very hot, light smoke from somewhere was the clue, but it actually did not ruin anything. Exhaust hoses were fine, impeller fine. Not that I'm reccomending to do that, but evidence that starting it dry for a few seconds won't hurt anything.
'06 X2 MCX
"I understand why some people may not want to do this the way I have recommended but I can't understand the death grip some people have on a toilet plunger with a hose fitting." -JimN