EU Ensign does not exist

One would hope that boat owners would understand that the ensign has nothing to do with the nationality of the owner.

There is no "EU ensign". It simply does not exist. The EU is not a nationality or a state, thank God.

1982 UN Convention on the Law of the Sea - Article 92. Status of ships.

1. Ships shall sail under the flag of one State only and, save in exceptional cases expressly provided for in international treaties or in this Convention, shall be subject to its exclusive jurisdiction on the high seas. A ship may not change its flag during a voyage or while in a port of call, save in the case of a real transfer of ownership or change of registry.
2. A ship which sails under the flags of two or more States, using them according to convenience, may not claim any of the nationalities in question with respect to any other State, and may be assimilated to a ship without nationality.

Wearing an ensign from a state the vessel is not registered in may negate the insurance. Given the eagerness of insurers to avoid paying.

If you want to advertise your nationality, EU "membership", golf club, church or football affiliations the port spreaders are the place, not the ensign staff.