Taking the high speed train from Rome to Naples. We need to get to Positano and need advice about best options for two people.
Ferry, private driver, book a private sail ?
You need to give the month of the year. There are very few ferries after October. Here's a discussion for Sorrento, only halfway to Positano. Note also that you cannot wake up in Positano and fly out of Rome the same day. Do you have a budget? Is it a honeymoon? How far (including vertical steps) is it to your hotel from the Positano port?
https://community.ricksteves.com/travel-forum/italy/naples-to-sorrento-train-or-private-driver
Positano tends to take smaller boats than other towns with ferry service. This can cause a rougher ride, and fewer seats for sale. It is unlikely you can go from Naples to Positano in one ride, but I haven't tried to look it up. This board has hundreds of posts on travel to Positano, search box top left, blue.