It is true that if seas are rough the ferries do not run. There is definitely a dock.
The other important consideration of how to arrive is where your hotel is. Our hotel (Eden Roc) was on the main road, so right in front of where the bus stops basically and would be a major hassle to get bags from the ferry up to our hotel.
If your hotel is at the beach/dock, bottom of the hill; it would be the opposite situation and leaving town by bus or taxi is going to be a pain compared to how easy it would be to get on a ferry.
When we were in Positano from October 10th - 14th a couple of years back I recall of the 5 days I saw ferries running on 2 or 3 of them only, but this is completely tide/weather dependent so impossible to forecast and it did off and on rain during a couple of days. Weather was wonderful other than the occasional rain though! The waves were present but not so rough, we actually rented a motor boat from the beach in Positano and took it out all days ourselves on a day when the ferries were not running.
if the ferry is not running toward Amalfi and points further it will not be running to Capri either.
The ferry boat from Sorrento to Capri by comparison seems to go as scheduled in most all normal weather/wave situations.
Getting there from Rome we took a bus to Sorrento, then stayed in Sorrento for a few nights and then took a local SITA Bus from Sorrento to Positano. You could do the same without the stop.
The bus to Sorrento was all locals except for us it seemed and made plenty of unscheduled stops and arrived at least an hour delayed because of them, was more humorous than annoying ; we enjoyed the local culture aspect of the ride.
I am sure trains are faster but at the time seemed more of a hassle to me as we were not in a big rush.
At the time we had luggage and just wanted to avoid the hassle of Termini Station so took a taxi to the bus station is Rome a little further out which I believe is where you have to go to get a bus from Rome to Sorrento.
It was very easily done and the bus had good storage for our luggage both to Sorrento and then on the local SITA bus as well.
On the way back from Positano we took a car service directly to Naples Airport, cost was reasonable, believe it was 110 euro door to door in a nice/luxury car.
A driver all the way from Rome would definitely be pricey though.