In reply to afx22:
I work for a Scottish ferry company, which isn’t Calmac. I can’t comment on their specific policies because I don’t know them but can make the following comments, which are pretty universal.
You can usually fit a bike on a packed car deck without too much issue as long as there are only one or two of you, issues occur when a whole peloton turns up unannounced.
Foot passengers usually get on without booking BUT there can be exceptions which you wouldn’t necessarily know about in advance. If there’s an event on an island (eg wedding, inter-isle football match, show etc) a usually quiet ferry can be fully booked for pax weeks in advance. Also as Scraggy Goat mentioned above you might get a safety certification issue or if a member of staff is absent and they can’t get a replacement in time, they can sometimes still sail but carrying fewer pax. These issues will crop up suddenly.
If you are booked and something unexpected happens (mechanical, weather, operational issue) we can call you, let you know and get you rebooked. If you aren’t booked, we don’t know about you.
If Calmac let you change your bookings at relatively short notice with no charge, I’d advise you to book. At least you’ll have something in the system, and it makes life much easier for the crew if they know how many folk they have onboard and they don’t have to go chasing around the deck collecting payments.
Have a fab time!