Just turn up
I did the Algeciras - Cueta crossing in Nov and returned in Dec. No problem at all with turning up without having booked. On each occasion I got straight on and there was stacks of free space available (car deck not even half full).
You will see agents at the side of the road selling tickets at least 100 miles before you even get to Algeciras. I bought mine in Algeciras town (you save 10% if you get a return), and I jumped straight on the next ferry.
On the way back I just rocked up at the port with my ticket (nothing booked) and got straight on the boat, door went up and we were underway in less than 5 minutes.
Really, it's a slick operation! And there is a boat every hour or so.