Small outboard engines run on fine margins due to their size and the single biggest problem with salt water engines is corrosion, it’s not practical to expect the average owner to fresh water flush their engine after each and every use but this is something owners should consider doing as often as practical and most certainly prior to any “lay-up” period.
If ever you are wondering “Why bother”, see the next 2 pictures, these are a Mariner 2hp 2 Stroke that the previous owner had not bothered to flush, basically the white rings are encrusted salt blocking the channels which cooling water should be passing through. A fresh water flush every now and again, especially before storage would have prevented this.
Unless your engine is air cooled, NEVER run your engine dry, not even for a few seconds, you will, without doubt damage your water pump impeller and could possibly end up with a large maintenance bill.
To fresh water flush an outboard is really quite simple, the best thing to do is to buy a cheap wheelie bin.
B & Q do one for £42 
Which is exactly what we use, fill it up with water, your engine will clamp on to it and there you have the perfect flushing tank which will last you for years and years.
There is no problem adding either a mild detergent to the water in the bin (Fairy liquid) or even a mild descaler to try to prolong the life of your engine.
Ensure that when your engine is clamped onto the bin that the water level in the bin is at least 10 cm above the water intake holes and then simply start and run the engine in neutral for a few minutes. If your engine has a thermostat you should run the engine for long enough to ensure the thermostat opens so that the whole of the cylinder block is flushed through. An example below from the Tohatsu 4 stroke 4-6 HP owners manual.
While you are flushing your engine, of course you are also looking to make sure there is the “normal” flow of cooling water through your engine, if you are unsure of where the water should be flowing from check in your owner’s manual.
Example below from an old Yamaha Owners manual
