Outboard vs. Inboard Flushing
Why flush an outboard when you don't flush a diesel engine? Because diesel engines, and many inboard gas engines, are fresh water cooled. The only parts that come in contact with seawater are the raw water pump, heat exchanger(s) and exhaust elbow, and those parts are typically made from galvanically noble, corrosion resistant materials, copper, copper-nickel, bronze, stainless steel, and they are replaceable. Outboards, on the other hand, are entirely raw water cooled and the powerheads (which are technically replaceable, albeit at great expense) are aluminum, a highly corrosion prone, galvanically ignoble metal. So while uncommon, flushing an outboard wouldn't hurt. For daily use it's probably less necessary. If, on the other hand, it was to remain unused for weeks or months, flushing would make more sense.