We had some friends (attempt to) take it yesterday. They're collecting fares now. You can buy a one-way or round trip in the mTicket app (no cash or paper tickets accepted). You can also use the monthly linkpass, but only the paper ticket version that's good for Commuter Rail Zone 1a too. This is what hung our friends up - they have passes on Charlie Cards which the operator didn't accept. Rather than buy ferry tickets, they just walked over to Maverick and took the Blue Line. This is definitely one of those obnoxious quirks of the T's fare collection system which hopefully will be fixed soon.