Final answer:
Water must reach a temperature above 25°C (77°F) to be suitable for warm water diving. The unique property of water being densest at +4°C causes a layer of warmer water to exist under the ice in frozen lakes, which is too cold for warm water diving but allows for aquatic life to survive.
Step-by-step explanation:
The temperature at which water must reach to be considered suitable for warm water diving is above 25°C (77°F). In many northern regions during winter, the surface of lakes may be frozen, yet there is a layer of water below the ice that stays at about 4°C to 5°C (39.2°F to 41°F), due to the unique properties of water. Although this temperature allows for aquatic life to survive under the ice, it is too cold for warm water diving.
Water exhibits a unique behavior where it is densest and has a maximum density at about +4°C. As water cools from warmer temperatures to 4°C, it becomes denser and sinks. However, if it cools below 4°C, it begins to expand again and stays on the surface which leads to ice formation on lakes and ponds. This characteristic creates an insulating layer of ice, fostering an environment for fish and other aquatic life to survive in the relatively constant temperature of water beneath the ice.
For scuba divers, understanding the temperature of water is critical, especially when considering the comfort and safety aspects. Warm water typically offers more pleasant diving conditions and requires less thermal protection than colder water. Hence, determining the appropriate temperature for diving is essential and is generally agreed to be above 25°C (77°F) for a diving experience to be classified as 'warm water diving.'