Electrolysis is what happens to metals when they are not protected by sacrificial anodes.
This sterndrive lives on a pontoon where the anodes are high and dry and the actual sterndrive was submersed in the salt water. As the anodes were unable to attract the electrodes produced by the salt water the casing of the sterndrive was attacked. Unprotected metals are no match for rampant electrodes. You can see the clean holes eaten into the metal as the electrons searched for and found completion behind the casing.
This drive either needed to be totally submersed in the water or totally washed down with fresh water and stored 100% out of the water.
Repairs to the sterndrive casing once this level of damage has occurred is $3,289 + Labour.