I have done the JFK 6 times so far, but keep in mind that everyone runs their own race, so what works for me may not work for anyone else. That being said:
1) Each aid station is different, since they are run by various groups (schools, boy scouts, etc) but in general they will have water, gatorade (or the like), soda, candy (M&Ms), pretzels, banana pieces, potato chips. Some will have other things like PB&J, soup, potato pieces, gels/gus. They will also have vaseline, motrin.
2) Aid stations are every couple of miles once you get off the AT. I usually carry a bottle on a belt, my first few times I started with a camelback and switched to a belt off the AT, but I found that I don't need to carry that much water. There are aid stations before the canal portion, they are just not as close together or regular. I also carry motrin, exceed tablets, moleskin, and extra socks.
Something you will have to decide about are shoes. If you have someone supporting you, you might want to consider switching between a trail shoe to a road shoe when you hit the canal portion. However, the AT portion is not that long and not that technical, so you can certainly get away with a light trail shoe the entire time (or even a road shoe).
The JFK is a well run and friendly race, you will have a great time!