I walked a lot when I lived in Manhattan. I took the picture below on one of my walks through East Side Park in 1991. It was a cold November day, and very clear.  The amphitheater was south of the park. As I recall it was even further south than the Fire Boat Station.

Graffiti Amphitheater

I don’t know the fate of this structure. It looks like this could have been a nice place for concerts or plays in its time. It is probably gone now.