Steel corrosion and stray currents

There seem to be competing ideas on grounding systems for steel hulls. But one article said only a single point of ground to the sea. So, you would need to isolate that ground and then check the hull to neg bus of each power system. If you have multiple points of ground to the hull; then there is a higher likelihood of ground seeking via the hull which for the situation you describe is very bad and a likely cause of the bubbles, etc.

 

The water has a higher oxygen content near the surface. Also, the wet/dry and cold/hot cycles are more pronounced near the waterline. All that is more conducive to corrosion.

 

Here is another set of tests to perform for ground problems. I’m rather surprised the forum has been so quiet on this issue.

Locating Ground Faults ("Current Leaks") and Short Circuits | Boating Mag https://www.boatingmag.com/how-to/locating-ground-faults-current-leaks-and-short-circuits/