Returning to the original issue of lower unit corrosion, I just found out yesterday that lower unit corrosion, and more importantly prop corrosion are almost inevitable, even in fresh water, and even with the zinc scrifical corrosion plate.
It even happens with stainless steel props, unless they have a high concentration of nickel.
The only way I was told to resist corrosion is to keep the prop/lower unit painted. In the case of the prop, the paint comes off the edges, the electrolysis starts and is seen even in the center of the blades where the paint is bubbling/blisering off.
Ah, the elements have a way to beat the best that man can muster