I have no experience yet of trailing my Baycruiser 20 abroad post Brexit, but as an owner of a, soon to be disposed of, SSR registered boat in Italy, can vouch for the fact that VAT checks on UK vessels, in or out of the water, are ramping up.
The local financial branch of the Italian police boarded me in Italian territorial waters a few months ago and I was able to satisfy them that I was not VAT liable locally largely because, unusually, my boat was purchased by its first owners in 1990s France and, thus, is VAT paid in the (current) EU. In fact, if you read the relevant RYA guidance - for addicts only - of itself, that does not absolve a non EU owner from paying VAT again in the EU, post Brexit transition, depending on when the boat reentered the EU from the UK and for how long.
A relatively clued up and efficient French official might have given me a harder time. Equally, I would not bet on the Dutch, or the Irish, turning a blind eye to the post Brexit EU wide requirements on trailing from the UK, once they have absorbed them. The relative freedom of the past 40 years to roam within the EU, without much paperwork, is history and we have to equip ourselves for the inevitable controls at the point of entry. What we need is clarity about the paperwork. The RYA has made a start.
In the last 2 years I have collected more passport stamps in the EU than in the preceding 40, globally!