There is a Dive New York forum under Northeastern U.S. with tons of members who can advise you. I also recommend contacting Scuba New York in Yonkers, as they are very knowledgeable about charters. I think they have a smaller shop somewhere in Brooklyn. I believe the charters in the ocean off L.I. South Shore get filled quickly in the summer. Doubt they'd start up before June, but am no expert. I also believe some leave early A.M., and don't forget good old NYC rush hour weekday traffic and South Shore summer weekend beach traffic. If you're crossing the L.I. bridges I think they're $8-$9 each way.
There's not good shore diving close, but if you're interested in that, google NY/NJ beach shore dives guide, or something like that. There are site reports, directions, etc.
If you plan any shore diving in the area (Conn. included), ESPECIALLY Long Island, make sure you can park legally close enough to the dive site. Rules change, often only locals with stickers can park, etc.--at times info. that is missing in the shore guide.
