You have a bad case of galvanic corrosion. The major things to check...
1) if the boat is on shore power
you probably need an isolator
2) is the boat bonded? To what
3) state of the anodes. Remember in brackish water
you need aluminium not zinc, you might also want to look at going to magnesium (fresh water) to help control the issue.
4) dissimilar metal contact. I doubt this is a problem because the corrosion is so rapid.