You can get the Twisto bus from Caen to Benouville and Pegasus Bridge is about a 10 minute walk from the bus stop in Benouville. Bus Verts are good but do not go via Pegasus Bridge that often. Good service to Merville. The museum is a about 15/20 minute walk from the stop on the coast.
Bus Vertes do a one day bus pass, La carte liberté. You can purchase these at the Ferry Terminal Tourist Office or on the bus. Twisto also do a one day family ticket and one day tickets, available at the tram stop machines. You can check out the timetables online.