Right next to
this restaurant is the Esplanade. It's a paved walkway along the water (on the BROOKLYN side of the East River), with a grassy area too. It has that classic view of the Brooklyn Bridge with lower Manhattan behind it.
Be prepared to work fast and possibly be chased. I was there long ago for a small video shoot and a cop sent us on our way. Just be charming and you might be able to stay. You might need a permit, but I'm not sure from whom. I suppose you could start with the Mayor's Office of Film & TV.
Anyway, use the
River Cafe as your landmark, and you'll be in the right spot.
I found this
picture to give you a general idea, but you would actually be slightly north of this point. Keep in mind when you're in Brooklyn looking at Manhattan, you're facing WEST (a.k.a. sunset!).
This
picture is more of what I'm talking about! This is the exact spot where I was.
You should also e-mail Chris Humphreys and check out
this thread. He got an AMAZING shot at night. But don't steal it!!!

Well THAT should be enough advice. Whew!