We've been to Sorrento 3 times, staying a week each time. It's a good amount of time to really enjoy it all. We've always been there in July/August. I hate high temps (I live a few miles north of SF) but Sorrento in the heat was not a problem for us (unlike Venice, Rome, Paris, London, even Tuscany in the heat). Being right on the water makes a difference, riding boats and the Hop-On Hop-Off bus where you get a breeze also helps. We stayed at La Tonnarella each time, they have A/C and a private beach which we loved. We bought large tubes in town and just floated around on the water, it was relaxing and fun. We also rented a small. private boat on Capri (not expensive - called Banana Boats) several times and spent the day driving around Capri stopping wherever we wanted to swim.
We've gotten to Sorrento from Naples by boat, circumvesuviana train and private driver. Ken's advice to take the bus would be my 1st choice, private driver my 2nd choice, and the train my 3rd choice. The boat with suitcases was a huge negative (long story), we had to wait 1 1/2 hrs for it while gypsies kept bothering us and the boat we took was filled with commuters who closed all the curtains so we couldn't see out the windows, and there was no where to go outside on the boat. So, we saw nothing and had to breathe the engine and gas fumes the whole way - not fun. The boat will bring you to the port in Sorrento and from there you'll have to take a taxi or bus to town. The circumvesuviana brings you right into town.