1. If you don’t require a cabin and have no bikes, then usually you can just walk on and pay.
2. If you don’t require a cabin and have bikes, then you have to prebook your bikes onboard (at least 48 hours in advance), and that can only be done manually by calling them. It’s highly unlikely they don’t have space for bikes, but it might be worth phoning a week ahead to check what they recommend.
3. If you require a cabin. You should book at least a few weeks in advance. In peak season, some boats are fully booked months in advance for standard cabins. The longer you leave it, the more likely only expensive cabins are left, and these are crazy prices.
Please let us know how you get on and how easy it was to get your bikes booked onboard with short notice. I perosnally have never had an issue with my bike. But good to hear your experience, so I can pass this on to others who ask.