The vessel must reenter the EU tariff zone from a foreign non-EU tariff place to be able to claim relief from VAT as a temporary
import.
All very straight forward, just like other goods. The relief is derived by being granted a waiver as a temporary
import from outside the EU. To be imported into the EU, a good must have first been exported from a country into the EU. Hence, international waters does not derive an export, hence no relief by temporary waiver for an import.
Just like departing to international waters does not avail a new cruising
license for voyaging in the waters of the
USA. You must return from a foreign port or place.
The UK has left the EU but not sure whether entering the UK reset the EU VAT clock during the remaining "transition" period.
Like all things Brexit, a confusing mess.