ferry/boat travel to Amalfi in March

My total guess knowing that the routes you are talking about are weather/sea dependent on a day to basis ; is that there is no such ferry that could be considered reliable for this route even if open at this time of year ; which I am not sure they are.
Positano and Amalfi have shallow water docks, so big waves means no ferries to these towns.

I don't think there's much running in March to either place but spring schedules aren't up yet so you could keep an eye on these websites:

https://www.capribooking.com/en/ferry-schedule?path_id=21
http://www.coopsantandrea.com/en/servizidilinea/servizi.asp

If the coopsantandrea site is showing blank pages, it means there's currently no ferry service to the chosen locations. In other cases, they show 2016 schedules so check dates carefully. A special note for Positano; even when ferries ARE running, they can't dock there on days that the seas are too rough. It happens less versus more often but it does happen. Same goes for Amalfi, it seems.