Thanks you MustRib.
I have a similar problem: mooring by the beach, children, diving . And a terrible experience.
I spend my summer holidays near a marine reserve.
Anchor mooring is not allowed in certain areas, to avoid damaging Posidonia (a sea plant).
Stopping the engine may be dangerous when close to cliffs.
So while picking up divers the engine should be at least at idle speed.
During Summer 2008 a 18 years old boy was killed by his boat (probably he forgot the security lanyard). We all saw it from the beach, at a very small village near Almeria. When they took it back to the sand he was absolutely bloodless. His name was Jose Manuel, I know his family.
20 knots to 16 knots and increasing a bit fuel consumption seems a low price. So I am considering the propeller.