I sail in the Long Island Sound as well (out of Mt. Sinai which is pretty far east) but have to disagree with the above poster(no offense, just my experience). I get great cell phone service while on the water. Sail mostly between Eaton's Neck (Huntington) on the western edge and Mattituck all the way to Connecticut and inbetween and usually can always make calls and or use the internet. We sleep on the boat 3-4 nights a week and mostly anchor out in different ports on the weekends and always use the iphone to check the local radar for thunderstorms. 9/10 times it works perfect. I have a nokia 6500 and an iphone (both at&t) and have no trouble with both.
I just recently broke my last iphone but am going to be getting the iphone 4 when it comes out later this month. I'm excited to try out your app.