Seconded. In fact, the first time I was in the ER was because I was scared by nonstop vomiting/diarrhea and they put me on an IV drip and then gave me a shot of compazine.

To check for dangerous dehydration, hold a pinch of skin/fat on your forearm for a second or two and then let go. If you're properly hydrated, the flesh will snap back to usual right away; if you're dehydrated, it will hold up in the wrinkle-shape of your pinch before going flat.