Dum spiro spero. Cras ingens iterabimus aequor.
I bought an SSR Registered boat in Portugal several years ago, and before signing the dotted line, I visited the Capitania in Faro to find out the regulations "straight from the horses mouth", and to avoid any unpleasant surprises afterwards.
Their only concern was payment of Light Duties which amounted to around 4€.per season.
Apparently, the Circulation Tax applied only to post 1986 boats with engines larger than 26.8HP.
Several months later, a Spanish charter boat dragged at anchor and damaged the side of my boat. He flatly refused to pay for the damage caused and so I had to refer to the Maritime Police. The first document that they asked to see was my 4€ Light Duty Certificate. None of my other pile of paperwork seemed to matter very much!