Take UK registered boat to Spain

Hello.

Assuming this is a pleasure craft on private use, not for charter.

If the boat has had the EU VAT paid for in any EU country (even UK prior to Brexit) you should be able to moor it there without any restrictions.

If the VAT has not been paid, you may have to move it out of the EU waters after 18 months, get some sort of evidence that the boat actually was outside the EU waters even for a day, and then get back into your Spanish mooring for another 18 months. I am not sure how many times can you keep doing this, but it would clear the issue for the initial 3 to 5 years. Of course, after a few years you may be better off just paying the VAT and not having to worry about moving the boat often at inconvenient times.

Regular customs VAT settlement is done with an assessment of the boat's value on the date of such payment regardless of the original acquisition cost.

About the actual place "outside EU waters", you can go to Gibraltar which is not too far from most Spanish Med ports.

I have heard of people who do nothing, stay on indefinitely and get away with it, but they could be approached by the customs and forced to pay VAT in terms that may not be the most pleasant.

When you arrive in the mooring port, you will be asked to produce the boat's registration and 3rd party insurance. Not VAT status.

Good luck.