There are 2 buses to Positano from Naples ( Porto Immacolatella) via Sorrento that leave at 8.45 and 9.10.
They run daily except Sunday and bank holiday, and they take about 2 hours to get to Positano.
I suggest you to take one of this two buses, then to visit the coast from Positano you could take another bus or a boat to Salerno.
It will be more easy and you will not stay to many hours on the buses, that especially in this part of the year is a nightmare!!!