Booking a seat gets you a reference and a link within a few minutes. The link shows your sailing, your seat count, what you paid and what to do at the jetty, and it keeps working after the trip so you have a record of it.
Show it at the counter or say the reference. There is nothing to print and nothing to collect the day before.
If you booked for four people it is one reference for the four of you, and the crew tick all four off together.