Education... photographic identification of electrolysis.
Fit the SeaBis System - its the most economic way for you to protect your installation.
Bilge pump.
This is very common. The wires to the bilge pump lay in the bilge moisture and insulation becomes conductive. Another very common cause is a crimp lug connecting the bilge pump lying in the bilge. Always lift wiring clear of the bilge moisture.
Note:
The semi-regular shape is very common. When sheet metal is produced by rollers, some areas become more electrically conductive - have a lower resistance to electric current - so electrolysis passes through those areas, to water, easier than the remainder of the sheet.