Where would you go diving near either SF or NY in August - October? I'm guessing the Caribbean is out because of potential hurricanes. Pacific Coast Mexico?
The Caribbean does pose the potential for hurricanes that time of year. However, if you were willing to book on shorter notice, or just like to gamble, it's a possibility. I parlayed a Playa del Carmen wedding into a spontaneous diving trip to Belize two summers ago during the first week of September and it was glorious. Bermuda, the Azores (off the coast of Portugal) or off the coast of Spain seem like cool possibilities that are out of the common hurricane route.