These are the latest times for the Supratours buses (as of March 2009)…
Marrakech to Essaouira – 08.00, 10.30, 14.30, 17.00, 18.30
Essaouira to Marrakech – 07.00, 09.30, 11.45, 15.30, 18.45
Buses can fill up quickly, especially in summer, so if possible, do book in advance. Unfortunately it’s not possible to do so online because www.oncf.ma still has no online booking system – so at extremely busy times you may need to turn up and reserve on the next-but-one service.