It was the reflection of the sun in an airplane's cockpit window. The airplane was actually maybe five kilometres away but the extreme brightness played with my sense of distance. The window must have been flat for the reflection to be so bright.

It didn't appear to move because its speed north relative to the bridge matched my speed south -- i.e. the line between me and the airplane crossed (just west of) the bridge at the same point for a long time. The point of reflection must've been tiny. But it was right on me. I suspect no-one else in the ferry saw what I saw. And it must be extraordinarily rare for the circumstances to be maintained for so long. The illusion disappeared after the ferry and airplane made their turns.