That is not exactly true although the rules are made that way. I and many, many others do not have to do that with out of EU registered recreational cruising boats and motor vehicles. There is no way enforcement can happen because there is no way to track them.
I have now kept my California motorcycle in Germany for 5 years. I have ridden it it 7 times over those 5 years there through 8 countries (some not EU), crossing borders several times in a day. I will be in Europe to ride it in a month, I do not pay tariffs or fees, i do not need to pass customs and I do not ask officials exemption. The law enforcement over the 23,000 miles I have ridden I have met only like to take photos sitting of themselves on my cool old bike. Most just give me a thumbs up. I have reason to believe they will do the same with a cool sail boat with a motor under 20 feet that is used less than 3 months a year.