Popular cheap method from UK if you are on a mooring or in a marina, lift your engine to full tilt, slide a plastic dustbin over the gearbox & bring it up the engine as far as you can, secure it with a rope over the top of the engine cover, drop the engine back down as far as you can, fill the dustbin up with fresh water & run the engine.

You may lose some water out of the drum by splashout, & it wont be as thorough as flushing with muffs or in a barrel, but if you are in a marina, or on a swing mooring, its better than leaving salt water in the water jacket. I have heard of people using the 1tonne sand container bags for the same pupose, but never witnessed it myself.
blueskip