Here where I live we have to winterize the engines every year, first we flush the seawater circuit with freshwater before filling it with coolant mixture to protect form corrosion and freezing.
Last year I found this device (Flush adapter for inboard engine
) making the process easier. It is an adapter to place over the seawater strainer (outside) and connects to a garden hose. This eliminates the need for taking the freshwater hose onboard.