The RYA are surely not qualified to advise on this subject.

This story has been doing the rounds of English language papers in Spain and the Telegraph seem to just be repeating it.

The 12% tax applies when a Spanish resident registers a boat.

A Spanish resident is someone who resides in Spain for over 182 days in a year. They are then liable to pay all taxes levied by Spain on their worldwide income, their wealth and any other taxes which apply to a tax resident. There is no choice, it all depends on the length of time spent in the country.

I am now tax resident in France, so needed to check these things out carefully.

I would suggest that the easiest way to prove that you have not overstayed would be to keep something like a creditcard statement which would show your location throughout the year.