Brexit has caused 2 issues:

Firstly, UK flagged vessels, EU registration VAT has to be paid on your vessel if it is being kept in the EU.

Secondly, UK boat licenses, currently due to Brexit UK licenses are not valid for using EU registered vessels, UK license holders can use UK flagged vessels in the EU. The Spanish authorities are adapting Spanish law to also allow UK boat license holders to pilot Spanish registered vessels. Due to the tourism this will be resolved soon though.