This paper presents an open-source watertight multirotor Unmanned Aerial Vehicle (UAV) capable of vertical take-off and landing on both solid terrain and water for environmental monitoring. The UAV's propulsion system has been designed so as to also enable the active control of the UAV's drift along the water surface. This low power locomotion method, novel to such a vehicle, aims to extend the available operation time on water bodies' surveys. The ability to take-off from water allows the UAV to overcome any obstruction that appears on its path, such as a dam. A set of field trials show the UAV's water-tightness, its take-off and landing capabilities in both land and water, and also the ability to actively control its on-surface drifting.