With the full implementation of BREXIT coming into force on 31/12/2021, it means that the UK will be treated as a third nation, in regards to travel for UK citizens, within the EU zone.
Basically, this limits travel within the EU to 90 days in any 180 day period. So if mooring holders decide to cruise to, say, France Belgium or the Netherlands, then, perhaps, on skiing holidays, or summer holidays to other EU destinations, they need to be aware of the 90 day limit.
Those who have holiday homes may easily fall foul of the regulations & will have to consider applying for additional visas to extend residency. Different countries within the EU may well have differing rules regarding this, so one needs to check with the host nation.
Of course all this may change if the UK does negotiate some different deal with the EU, but a change to the 90 day rule seems unlikely, as the UK is not showing willingness to offer reciprocal deals in this matter.
To assist in calculating days available, the following link to a schengan calculator is given below. On the calculator page there is a further link (bottom right) to some advice on its use & the 90 day rule. We advise those who may get near to the 90 days to read this.
As additional info, here is a link to government advice on travel insurance