I helped with the coach boat for my wife's Dragon Boat team, they had newish Merc 9.9s that had an external circuit that you could disconnect and hook a regular garden hose up to it for flushing. The connector was just beside/outside the cowling, up front opposite the tiller arm.
Worked slick but I don't know if it's standard nowadays or if it was an add-on.
What year is this motor???
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)