With Brexit, British crew members are no longer circulating freely within the Shengen area.
They can enter the EU without a visa and stay for 90 days within a 180-day period. They get a stamp in their passport when they enter the EU and that will start the clock on the 90 days.
When they join a boat, they can request to be stamped out of the EU. While they work on board, it stops the count of the 90 days.
When they disembark, they need to request to be stamped back in the EU.
While crew are part of the crew list and stamped out of the EU, most EU countries will allow them to travel within 10 kms of the boat.
Any travel further than that will require a stamp back into the EU.
British crew who have been embarked before Brexit do not need to visit immigration to stamp out of the EU. But when they disembark or travel beyond the 10 kms, they will need to stamp in the EU and their 90 days in the EU will start then.
Disclosure: This is not a legal advice and the rules are subject to change at any moment.