I visited Positano in mid of August with my husband and took a private boat tour to Capri from Positano. We booked Romar boat before our arrival at Positano. Our tour started around 10 AM and we were back around 4:30PM, though we booked a 7-hour full day tour, but we were very content with the 6 hour ish ride. The boat has basic amenities like Towel, life jacket and we were also offered drinks including water, Coke, Beer. Our sailor for the day was very nice and kind. He explained us very well and showed all the beautiful caves around Capri island. We also stopped at one of the caves to swim.
I would recommend their boat service.
P.S.: It is good to reserve a table at Capri island before going, though the Grassi Junior guys can also arrange one. But I would recommend reserving your own and tell the sailor so he can drop off you at the Capri island at nearby location.More