noun - A heat is a specific group of individuals who start at the same time in an open water swimming competition. Individuals in specific waves can be determined by gender, age, speed, ability or distance or course to be swum. Waves are usually started in different intervals that can generally range from 1-10 minutes. At very large races, racers may be finishing the entire race when the last wave begins their swim or even on different days.
The young boy and his father were in different waves at the start of the triathlon.