Although the GoSun is capable of boiling water, the GoSun BREW is recommended as an additional accessory to heat up any type of liquids and also for boiling water. The BREW acts as a separate capsule which slides into the GoSun tube, and protects the device from any potential thermal shock. More news to come on the BREW accessory as it becomes commercially available! (ETA is Fall 2016)
Keep in mind - GoSun will not rapidly boil water like a standard stovetop. Instead, it will slowly bring water to boil (approx. one hour of full sunlight for 1-2 cups of coffee). GoSun actually cooks food much more rapidly than it takes to boil water, because of the comparatively high density of liquids over food.