I would also book.
We used it last year to get to the Harwich-Hoek ferry and it was pretty full so book a space to avoid having to wait until the next crossing.
Also, we intended to use it yesterday having camped in a pub near Harwich on Saturday night only to discover they had made the decision to cancel all crossings on account of the expected high south westerly winds. If you have booked I understand they attempt to make contact to let you know of such issues.
Fortunately we had been on the website the evening before so knew it wasn't running but that didn't improve our return journey from Harwich, largely into the actual strong south westerly winds as we went back the long way round to Ipswich!