Hypothermia can set in after someone is submerged in water as warm as the high 70s.
PHOENIX (AZFamily) — It happens several times every year, according to Sgt. Robert Marske, who works for the Maricopa County Sheriff’s Office. Marske investigated boating accidents and incidents for 16 years, and he says springtime can be dangerous on the water.
Source: Hypothermia still a danger in Arizona lakes despite warmer weather