You can, but it's not necessary. For first time use, you may need to flush the solar shower with hot water. But it doesn't necessarily have to be boiling water. The purpose of solar showers, unlike other classic models, is for you to have on-site heater water. Since boiling water tends to cool after a few hours of hike, this may not be advisable. But boiling water should in no way affect the material or cause leakages.