Thanks for the good intentions,
but that tietable is no longer valid.
Metro del Mare no longer seems to exist
and currently there is no direct scheduled boat service between Napoli and Positano.
Our attempt to find an onwards connection by taking the boat to Sorrento failed (although there might be connections that can not be booked through earlier in the day) and resulted in high taxi costs.
In case any one else wants to try the best possibility seems to be connecting via Capri (although this also can not be booked together and in case of failure you are caught on a very nice but expensive island).