When former NFL quarterback Ryan Mallett was pulled unresponsive from the water off a Destin, Fla., beach Tuesday and pronounced dead a short time later, many assumed he had fallen victim to the deadly rip currents which had already claimed at least a dozen lives along Florida Panhandle beaches.

But officials say that isn’t the case.

Yellow flags were flying Tuesday along Destin beaches, meaning only moderate surf or currents. At 6-6, 245 pounds, and only five years removed from his NFL career, Mallett was still in good shape, based on photos from White Hall (Ark.) High School, where he had been coaching since last year. The water conditions should have posed no problem for Mallett.

“It just seems to be a tragic accident and not something to do with the conditions of the surf, the tides or the currents,” Okaloosa County Sheriff Eric Arden told WEAR.

So what killed Mallett? Repeated calls to the Okaloosa County Medical Examiner’s Office went unreturned, but Destin Beach Safety Division Chief Joe D’Agostino said the extreme heat may have been a factor in Mallett’s death, according to the outlet.

Indeed, water temperatures which currently range from 82.2 degrees off Panama City Beach to 86.5 degrees at Coast Guard Station Sector Mobile, according to NOAA, coupled with heat indices which have soared well into triple digits this week, are a deadly combination.

In October 2010, American world-class long distance swimmer Fran Crippen died during an open-water race in the United Arab Emirates, with water temperatures in the mid- to high 80s, similar to the current conditions along Florida Panhandle and Alabama beaches. The average high there in October is 93, according to Weather Atlas. The temperature in Destin Tuesday was 96.

Several other swimmers were treated for heat exhaustion that day. An average of more than 600 people die in the U.S. each year from extreme heat complications, according to the U.S. Centers for Disease Control and Prevention.

Beachgoers entering the water, regardless of the flags posted, should be aware of the dangers of swimming in extreme heat. People tend to be under the mistaken idea that being in the water brings them relief from extreme air temperatures, but heat exhaustion or even heat stroke remain a possibility when water temperatures are also high.

Heat stroke occurs when the body can no longer control its temperature. The body’s temperature rises rapidly, sweating ceases and the body is unable to cool down. The body temperature can rise to 106 degrees or higher within 10-15 minutes, according to the CDC.

Symptoms of heat stroke include confusion, altered mental status or slurred speech; loss of consciousness; hot, dry skin or profuse sweating; seizures; and very high body temperature. Heat stroke is fatal if prompt treatment is not received.

Heat exhaustion is the physical response to excessive loss of water and salt, typically through excessive sweating. People with high blood pressure, working in a hot environment and the elderly are particularly susceptible.

Symptoms of heat exhaustion include headache, nausea, dizziness, weakness, irritability, thirst, heavy sweating, elevated body temperature and decreased urine output. Heat exhaustion can escalate to heat stroke if medical attention is not administered.

Medical experts say swimmers should be aware of the risks of dehydration when in the water and, again, especially in the current warm water and hot air temperature conditions.

People who are active in the water can forget they are still sweating, which can lead to dehydration. Experts say it’s important to drink plenty of water both before and after a swimming.