There are 3 ramps that I am aware of, to launch in Jamaica Bay:
1 - Nassau County Park - Bayview Avenue. This is a very good ramp, plenty of water at all tides and near the fish. Only issue is that it is a Nassau County Park and Requires a Nassau County Ramp Permit. The park manager is nearly always on duty and he WILL come by and see you when you drive in. You will have a problem with out the sticker.
2 - Town of Hempstead Marine Ramp - Inwood. There is town of Hempstead marina at the back of Jamaica Bay that has a ramp. I don't think you need any kind of permit here, but the end of the ramp has a sharp drop-off that is dry and dead low tide. You would only be able to launch and recover here at mid to high tide. Good news is no permit needed (I think).
3 - There is a ramp along the seawall just west of the cross bay bridge. I have never used it but am told it is a good steep ramp, just be careful with what you leave behind in your car at the parking lot. I'm not sure on the permit requirements. Hopefully someone here is more familiar with that particular ramp and can offer up some more info.