While docked in Bergen, can you just walk up and purchase a ticket for the fjord cruise to Mostraumen, or do you need to book in advance?
I do not how many possible operators but ‘Rødne Fjord Cruise’ has a three hour round trip daily at 10:00 and at 14:00. No stops or going ashore during the tour itself.
For a guaranteed ticket, book at rodne.no. Also available for purchase at the pier/boat or at the Tourist Information.