Dsis did a river cruise on the Danube last year which took them through 5 Countries. Passports were checked at beginning of cruise and thereafter they crossed borders (sometimes during the night) with no further checks necessary. Dsis asked the guide what would happen if the UK brexited. The guide said it was simple - they would no longer advertise these cruises to UK citizens, as having them on board would entail going through customs checks at every border, which would seriously reduce cruising time and inconvenience other EU nationals who did not require border checks. It was already the case that they did not accept USA, Japanese, etc tourists, only EU citizens to avoid this problem.