I do not really think there is a problem with Schuko or 13 amp sockets, I know my son's boat had a warning lamp to say if reverse polarity, and his shore supply only powered a class II battery charger, all the boat's 230 volt sockets were supplied from the inverter, so the shore supplies earth did not connect to the boat.
However, if inspected, it depends on how pedantic the inspector is, Schuko sockets are not permitted in the UK, you can have Schuko plugs, and 13 amp to Schuko adaptors, but not a fixed Schuko socket to comply with BS7671.
BS7671 also requires the use of the blue plug and socket for the main supply to the boat, I am not sure about the RCD, poor name as easy to get recreational craft directive (RCD) and residual current device (RCD) mixed up. He fell foul of the RCD with his AGA stove, and I could see no problem with it, and the use of the diodes in the earth I know are required, but it goes against the grain. The idea is you can have 1.2 volts before any earth current flows, so it does not damage the hull, I see you have it fitted
but with a plastic hull not sure if it is required or wanted? The idea with a metal hull is the hull as a very good earth, not so with a plastic hull, and since piloting a narrow boat seems to be a contact sport, not sure, I like the idea of a plastic hull.
I would suggest going on a narrow boat forum, as it is really a specialist subject.