I was diving in Monterey a while back, and we had an all-hands type emergency response to the far side of the beach near the launch ramp. At least 3 police cars, an ambulance, 2 Fire Dept rigs, a small FD rescue unit, the Monterey Fire Boat, and the Coast Guard scrambled their 47' MLB. After much standing around (or motoring around, in the case of the CG and Fireboat), everyone dispersed.
I joked to a friend of mine, wondering if it was an acquaintance of ours, who was trying kayak diving for the first time. Turned out it was. Someone on shore had seen him capsize his kayak, and watched him having problems getting back on board. They called 911 (our emergency number in the US), reporting a drowning kayaker. All that despite the fact the guy was aout 50 yards from shore on a calm day, was in full dive exposure protection, and was probably in about 15 feet of water. He was back on board by the time the CG reached him. He could have swum to shore inthe same time.
Bottom line: it's not always an emergency when someone goes in the water. A little discretion before calling can save a lot of hassle.