Flushing raw water system with fresh water

[kapnd]

Leave the seacock open during flushing, and there’ll be no danger of flooding through the exhaust. Before the engine is started, and after it has stopped, excess fresh water will flow harmlessly out of the raw water intake.
As long as a decent size water hose is used, and dock water pressure is reasonable, and motor is run at idle speed, it will not be drawing any salt water thru the intake, and the strainer and intake will get a backflush after the motor is turned off.
On my boat, the Motor, head, deck wash and bait well are fed from the same large intake, so after shutting down the motor, I can flush these systems with fresh as well, eliminating the rotten egg odor syndrome!

[bongo]

On merchant vessels that always operate in salt water, sea life builds-up and eventually clogs everything up. It can be terrible, with mussels in all of the salt water service sea chests/strainers/piping... That same vessel, if it sails into a fresh water port for half a day every few weeks, will stay clean as a whistle. Critters that thrive in salt water don’t do so well in fresh water, and vice versa. If the vessel has been in salt water for a very long period of time however, the fresh water flush can be hazardous, as the accumulated salt water critters all die at once and clog the piping/heat exchangers... Don’t ask me how I know this��
Of course on ships, these water systems are circulating water 24/7/365.

[BobHorn]

Sounds like one of those things where the cure is worse than the disease.

[parkstone bay]

If the set-up is already in your boat, then use it. As previous posters have said, usea bucket as near as possible to the same level as the raw water intake (so that 1. the engine pump is doing "roughly" the same work, 2. you're not over- pressuring the system using mains pressure). it'll take a few minutes adjustment of the mains tap to keep the bucket topped to the correct level (that is, quantity of water in = water out: if it's too low, you will empty the bucket, and eventually the pump will run dry, engine overheat etc; if it's too high, you fill the bilge with fresh water). I flush mine once/twice a season. In the winter time, it's a good way of getting some antifreeze/inhibitor into the engine. But, of course, many people just don't bother at all.