What must be added to a dry suit for thermal insulation?

The requirement for thermal insulation in a dry suit diving context hinges on maintaining body warmth while in cold water conditions. An undersuit or liner is specifically designed to provide thermal insulation by trapping warmth close to the body. This is essential because, while a dry suit prevents water from entering and thus protects against hypothermia, it does not provide insulation by itself.

The undersuit functions as an extra layer that helps to keep the diver warm, taking advantage of materials that retain heat even when damp. This is crucial in cold water environments where heat loss can occur quickly.

While other options may provide benefits in specific circumstances—such as buoyancy compensators aiding buoyancy control, weight belts adjusting for necessary weight to dive, and hooded vests offering additional head warmth—none of them directly provide the thermal insulation required when wearing a dry suit. The undersuit or liner is the dedicated solution for this necessity, ensuring comfort and safety during the dive.