As said above plenty engines never see a flush & keep running.
I got round the problem of flushing in a housing estate by using a "J- cloth" (Other dishwashing flannels are available!)
- Take the cloth - stuff it up the inside of the prop.
- Take a square (or any offcut of sheet wood you have lying about, as long as it's bigger than the hub & not woosy thin) then a length of bungee round the front of the gearbox to hold it in place. (make sure the wood isn't touching the hub or it gets REALLY noisy!
- switch on & flush, remembering of course not to engage gear!
Granted I do it in the first available afternoon, as I'm usually so gubbed by the time I get home flushing is about as far from the sofa & a malt than it could get, but the cloth (use 2 for a big prop) does muffle the noise to the point of similar to in the water.
I'm probably also helped by the Harley owning guy a few doors down who shares his exhaust nose with the whole town!