We plan to use several ferries between 8-16th June in the Dubrovnik area.
Is it necessary to prebook any of them and where?

Thursday 8th June Jadrolinja : Dubrovnik - Lopud Depart 8pm
Saturday 10th Alpex : Lopud to Sobra (Mljet). Depart 9.35am.
Wednesday 14th Jadrolinja : Sobra to Prapratno 9:00am and 16:00 p.m.

Finally If we caught the ferry to Prapratno, how regular are the buses into Dubrovnik?
Thanks

You can not pre-book local ferries just turn up and buy your ticket from the ticket office.  Unless things have changed since I was last there it is a long walk up the hill from the sea at Prapratno to the road where the buses pass along. You might have to get to Ston to catch a bus. The campsite at Prapratno should have the bus times and you may be able to find someone to give you a lift up to the road.  The only timetable thatt I know of is here http://www.libertasdubrovnik.hr/RASPORED%20VOZNJE%20PRIGRADSKE.pdf