Undergarments
By itself, a drysuit won't keep the diver warm. However, varying layers of insulation (or undergarments) can be worn under the drysuit to keep the diver warm. The primary advantage of a drysuit from this perspective is that it allows you to vary your insulation with the needs of the dive. The colder the water, and the deeper the dive, the more undergarments you will want to wear under your drysuit.
