Looking at the Washington sectional, several areas offshore are designated "Warning" areas: W-386, W-107, W-105 etc. The only notes given are "National Defense Operating Areas. Operations hazardous to the flight of aircraft conducted within these areas".
That would be enough to keep me well ashore, but out of morbid curiosity, what kind of activities are we talking about? Are these areas designated for naval gunnery/missile ranges?