This is a great thread!
I pretty much agree with DUNEDAIN with respect to New York accents.
I think there is a kind of "generic" NY accent that overspreads the city and at least northern NJ. That's the accent I have.
Then there are places where the NY accent seems much stronger. As DUNEDAIN mentioned, the Staten Island accent is different, but I also think that the Brooklyn accent is different than SI. I'll add to this that I think the strongest of all the NY metro accents is the Queens/Long Island accent.
If you're interested in American accents, you'll find that the greatest number of different accents occur in the Northeast and Mid Atlantic states, and there are historical reasons for this that are really interesting.
As far as Seinfeld, I was never really a fan (I like what I've seen of the show, I'm just not a TV person) but from what I remember, Jerry (who is from Long Island in real life) really doesn't have a LI accent; it's more of the "generic" NY accent. From my recollection, Elaine had a pretty light, generic NY accent. Her accent would fit well into northern NJ. Jerry's parents, however, had solid Brooklyn accents (from what I remember).
In regards to the "caught/cot" distinction mentioned my SNUGGLEBUNNY, that is entirely regional. I would NEVER pronounce them the same way.
The vowel sound in "caught" would sound like "awe," ("cawt") and the vowel sound in "cot" would sound like "hot" or "pot."