Bill came over and said the cockpit drain valves have to come out
The dezincification happens when your boats metal bits act as electrodes and pass a current through the salt water. There needs to be sacrificial anodes all connected together electrically (earthed) on your boat to allow them to decay rather than your metalwork.
Also check that your shore power has a galvanic isolator in the power line, this is a key component if you aim to use shore-power in marinas, if you haven’t have a chat with your mateys at PBO, they ran a self build project on one a while back.
Are the skin fittings ‘pinked’ also?