Unlike the subterranean termites, drywood termites form colonies within the wood itself rather than in the soil below. Since drywood termites form colonies within dead wood, they have no access to free water. This is also the reason for their common name: drywood termites.
This is also why they must either acquire water through metabolism of wood or through moisture in their environments. Some drywood termites make colonies in hot, dry areas such as Southern California, where high temperatures and low humidity reduce the available water from the environment.