In other cases, bait fish or blood in the water will naturally attract sharks. Some reported shark attacks were actually large schools of blue fish in a feeding frenzy. The most serious shark attacks have occurred in deep water when divers are exploring known shark-invested waters. People sometimes confuse shark and dolphin sightings. They can be distinguished by their swimming habit.
Dolphins have a curious arcing motion, diving up and down as they swim.
Water quality concerns all swimmers, and storm-water drains on beaches should be avoided. The Surfrider Foundation has been fighting such water pollution problems for years in Southern California with much success. Red tide is a very serious water quality problem, and the appearance of an algal bloom means that everyone should vacate the beach. These toxic algae can cause massive fish kills, and shellfish are poisonous to eat when infected. The Gulf of Mexico is regularly affected by these dreaded red tides, especially during spring to early summer.