Thank you, Ian, for your expert input on this matter. Personal experiences are very valuable.
I'd like to ask you many questions, but I'm afraid of being intrusive.

As far as I know there are 3 scenarios, sea water, brackish water and freshwater.

Zinc for seawater
Aluminium works for both sea and brackish water
Magnesium for fresh water

No one has mentioned the fact that different anodes are used in fresh water versus sea was.

My experience, in the marine industry, with sacrificial metal normally is Zinc and these are in the form of anodes found on the outside of ships and on smaller craft attached to the prop shaft. The idea any free current in the area will eat these away and leave the likes of expensive alli bronze props free. On ships they are stratically placed around the aft end where the props and rudders are. One will see them attached to the legs of out board motors and are easily identified as they are not painted. Hope this helps. Cheers Ian

What Chumm said, plus, anodes also protect the different metals of a boat below the waterline. Dissimilar metals, bronze, stainless steel, steel, aluminum in a saline, or even fresh water environment, where Magnesium is the anode of choice, constitute a battery. Not to mention "stray currents, as Chumm mentioned, especially for boats hooked up to shore power,(AC).

In any harbor where boats are docked or moored there are many stray low voltage electrical currents in the water. Sacrificial zinc anodes also help protect against these anomalies.

Hi JockScott, you explained it very well.

I have known about sacraficial Zinc Anodes be used on parts of vessels around the vicinity of the propeller as the slow process of electrolyse develops between 2 different metals (steel of the hull and bronze of the propeller). Over time they will be eaten away and have to be replaced to prevent deterioration of the propeller. Somebody with the knowledge of chemical galvanization processes would be better capable of explaining it in detail.
Placing copper sheathing on the wooden hull is a different matter and helps keeping mussels and algae attaching itself to the hull.

Well done, Pat and Dave.

Many modelers have attempted a task requiring enormous patience, both complicated and delicate. That is, covering the bottoms of various sailing ships (such as the Thermopylae and the Cutty Sark) with real copper plates. It is certainly the most extensive use of this metal that can be made in static modeling.
It mirrors the use of copper on real hulls for exactly the reasons you mentioned.

I had expected that in this forum (frequented primarily by dynamic RC modelers) the answer would concern the use of electrical cables.

Thanks a lot Chumm, we look forward to some more contributions.

I believe the original purpose of copper sheathing on the bottom of sailing vessels was for protection from marine fouling including the teredo worms that would burrow into the planking. Copper sulfate was originally included in ablative bottom paints. Very efficient for killing barnacles, etc but unfortunately also very efficient at killing other beneficial bottom dwelling marine species such as clams & scallops. Thus copper sulfate in bottom paints became prohibited in the U.S.
Zinc is the commonly used material for sacrificial anodes to prevent galvanic corrosion of underwater metals.

Let's leave some time for others who might want to respond.

A sacrificial metal use usually used to combat electrolysis thereby protecting more important metal objects from corroding - the most common metal used is zinc

Copper is used in very special circumstances and is used combined with another rarer more expensive metal - I will be honest thought I am not sure whether I did not know or have forgot where this would be used and I will not cheat !

Back in the day British ships were sheathed in copper to improve speed and stop the evil worm boring into the wood of the ship