I don't really go by water temp - in fact I don't even have a water temp gauge in my boat. This is because where I fish the surface temp that a sensor would pick up bears no real relationship with the bottom water temp.
Probably for surface-feeding fish like pelagics the temp up high means something, but not for the fluke, seabass and blackfish I look for. Even into the third week of June, with air temps in the 80's, fish can frequently come aboard that feel like they've spent the night in a refrigerator.
Anyway, from what I recall, the Sound fishing from Port Jeff out towards Horton's Point typically kicked off around Memorial Day. But from what I have read, even that is nowhere near a sure thing - not the past 6 or so years. Its been a bit of a tooth-pull putting together a nice catch up there. Maybe the newly proposed 18.5" limit will help a bit.
In my area, south of JI, we used to slam the fluke on the open sand right around then as well. No more - at least not the past four or so seasons. Now the really good fishing by me doesn't get rolling until the second/third week of June, and some seasons even later. I guess if I fished up in the Bay's shallows there could be some decent fish to be found. But I'm not a fan of boat traffic, narrow channels and tons of seaweed-clogged rigs. Too frustrating.
So I don't know if that helped - probably not. But at least I gave you a "when" if not an actual temp.
Oh, one more thing - the really old-timers used to tell me that a very cold Winter was conducive toward a big sand eel population the following Spring/Summer. That actually seems to be true, based on my observations.