We do not and can not diagnose from a video or a picture, but can offer suggestions.
From the pattern of corrosion, it looks a lot like a stray current problem from your videos. If so, it could be caused not only from your boat, but any boat around you or from dockside current leakage into the water. Automotive battery chargers (not marine) have been known to cause that type of corrosion if left on for long periods.
It would be to your advantage to enlist the services of a surveyor with corrosion experience or some company that specializes in the marine corrosion field.
The following links may help explain the types and causes of corrosion.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.