I took ferries there more than once and as they round that south-east corner of STT, they usually cut way too close to shore or rocks, in fact they tend to take a narrower channel closer to the island between the rock and the island whereas official channel is on the other side.
GPS? I've never seen these guys use any type of electronic navigation aid, not even a paper chart. They drive that damn ferry
as they please, 100 ft from rocky shoreline at 35 kts
Also, it was a day of festivities - so I have a feeling captain may have partaken in the juice.