Ferries hop from Brestova on the mainland to Porozina; and from Valbiska on Krk to Merag. Both take 30 minutes. In summer a daily catamaran connects with Rijeka (1hr 20mins) and Mali Lošinj, all going into Cres town, some then on to Martinšćica.
At least five buses a day run between Cres town and Mali Lošinj, two linking with Zagreb. Summer weekdays, some six buses a day run between Cres town and Osor, significantly fewer at weekends and also in winter.
Transport for north Cres is pretty scarce. You should manage to find taxis at Cres town bus station.