Galvanic corrosion protection

Yes magnesium in fresh water but Zinc andodes are fine on a clean hull in salt water.

You may be able to get an alloy base plate with 316 studs that you weld onto the hull, some are cast and some are tapped.
Use a zinc with a steel strap and drill the strap to suit the studs. Use stainless spring washers and nuts.
We usually recommend that the straps and the tightened nuts and studs be painted with epoxy since it reduces the anode corrosion rate to hide the bare metals from the sea water. Painting the backs of the anodes with epoxy is also a good idea.

Stainless would only ever be a problem if the anode were completely depleted, but by then the steel strap will also be problematic.

There are both zinc and magnesium anodes for outboards, the Mercathode system has had some abysmal failures and the alloy used for casting legs is a very poor alloy for marine use.

Passive protection is always significantly more reliable than active systems and since a boat still needs slipping regulalry it's not much extra cost to keep the anodes up to scratch.

Corrosion rates are not just about the level of nobility but also about ratios of areas.