how to prevent stray current corrosion from other vessels and shore wiring.?

Answer 1

It depends on the size of the problem.
As the previous answer, zinc anodes can be used but they will require constant checking to ensure they don't decay altogether. The second option if the system is larger would be an impressed current cathodic protection system.
Impressed current systems generate a dc voltage in opposition to the existing current direction from an inert anode(s) usually platinum coated titanium. Platinum degenerates very slowly when used as an anode and can last 30 yrs or more.

Answer 2

Zinc anodes fastened to all below surface metal. The zinc corrodes first. It must be replaced yearly.