I am not convinced flushing does any good.
In 1995 Alan Priddy organised a rib camping holiday to the Channel Islands. 10 days spent living on board for some of us whilst the rest lived it up in hotels. When we got back, the rib with a Yam 115 V4 outboard was towed up to the Thames near Lechlade for a weeks holiday. The rib spent most of each day going up and down the Thames mostly keeping to the speed limits after a couple of lock keepers timed us between the locks and earned us a quick slap on the wrist
After returning home again it was time for a good service. Out came the termostats and astonishingly they were still covered in large salt crystals. A week of ploughing up and down the Thames had done nothing to flush them out.
I became less fussy about flushing outboards after that.