Sailors Snug Harbor is a great place -- underutilized back when I was living in NYC, and perhaps still today. Great spot for a picnic in the summer. Should be a great venue for outdoor concerts at that time of year. No doubt chilly with the wind blowing off the water in January. Herman Melville's broother was the governor of the facility back in the 19th cent.

There is still a reasonably large contingent in NYC of people who enjoy OT and bluegrass music but I don't think, as you suggest, that many are on Mudcat. Back in the 70s and early 80s, the old Eagle Tavern on 14th Street got good turnouts for OT jams on Thursday nights and OT/BG music on Saturdays. But I haven't lived in NYC for more than a decade, and I don't know where that music hangs out nowadays.