Hallo
It is not possible to book the ferryboat, and normely there places on the camps.
I can recomed you to go to Daedalos Camping at Skala Sotiros. I live there myself in the summertime, so maybe se you next year.
You do not need to pre-book a ferry there is one at least every hour.
There are lots of camp sites and turning up is easy peasy, just don't turn up at Siesta time no-one to be found in my experience. Camping Rachoni and Golden beach are great camp sites, also Potos and Maries areas.
