Salt and brackish water is corrosive to aluminum outboard cases, so flushing every time after saltwater use is a must. Left inside the cooling passages, saltwater will leave deposits which can quickly build up and may cause cooling blockages, leading to overheating and, over time, can corrode an engine from the inside out.
All outboard manufacturers recommend flushing (according to the procedures outlined in the engine owner's manual) after every use in salt, brackish, or dirty waters. Operating an engine in sandy, silty, or muddy fresh water is also reason for flushing.

Flush muffs are the most common way to flush an outboard. Simply connect the inlet fitting on the muff to a garden hose, fit the muffs over the engine's water intakes on the sides of the gear case, turn the water on, start the engine, and run it at a fast idle (maximum of 1000 rpm for 10 minutes). Note that every engine manufacturer provides their own specifics, and instructions can vary according to the horsepower, so refer to your engine's owner's manual.

Inside length: 9"; see diagram

Note -
If your engine has additional auxiliary water intakes that are not being directly fed water via the flushing muffs, they must be sealed off with duct tape, or overheating may occur.
If your engine doesn't have water intakes on the sides of the gear case, you will need a special type of flushing attachment that covers the front of the gear case.
When attaching the muffs, be sure they cover the water inlets completely and don't pop or slide off when the water is turned on.
Never leave the engine unattended while flushing. Watch the engine's "tell-tale" overboard water indicator to ensure that the engine is pumping water.
Ensure that all water is drained from the engine after flushing. This is especially important in freezing climates, so that there is no water left inside the engine that could freeze and cause damage.