Short answer: No. Flush all you want. Only if water flows "uphill" in your area would you have a concern with flushing an outboard engine with the engine either running or not.
Exception: IF your exhaust baffle area has a leaking gasket that seals against the exhaust cover OR you have a cracked cylinder block or sleeve, you could see water intrusion to the combustion chambers/cylinders while using a flushing hose.
Persona Watercraft (SeaDoos, Waverunners, etc...) have a water cooled exhaust pipe that requires the engine to be running to push the water/exhaust mixture out. If you flush them (the older 2-strokes anyway) you can fill the exhaust system with water and can flood the engine with water.
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