Korean spa
Premier 57 is in Manhattan... on 57th, near the Park. It's really nice, and a pretty cool location. It's perched at the top of a building. Some of the spas are on the open air roof. But, the bulk of the facility is co-ed and clothed. There is a mens area with steam/sauna, showers, and individual jet pools.
There is also a Russian Spa, which I've heard is real rustic and quite authentic... but not really tourist friendly. They have separate mens/womens days.
just announced! Spa Castle is closed indefinitely, no explanation given on the website except that they hope to re-open.
The Russian and Turkish Baths on East 10th St. is an option although it is very different from a Korean spa. Several hot rooms and a cold plunge but not warm or hot baths like at a Korean spa.
By coincidence, I had to drive to College Point yesterday. So I decided to drive past Spa Castle to see if anything could be learned about their closure. On the the front door there were building permits issued by the City of New York. The permits were issued this week and were for moving (removing?) gas fired kitchen equipment, something about a stairway, and moving a sprinkler.