I have access to Cedar Beach in Mt. Sinai and I can get a day pass for access to Reeves beach out east http://www.aquaexplorers.com/shipwreckroanokepoint.htm
I also joined LIDA and working on getting the seasonal pass to park by the Ponquogue Bridge which i haven’t dove yet but want to. That area you need to dive with someone who’s done it because of the currents. I hear as long as you dive the end of the incoming and slack tide you’ll be fine but it would be nice to dive with someone who knows the area.
I also know an area in Stony Brook in the back bays.