The reason that it’s so rare to see footage like this is because there are simply not many of these planes around. Only 90 CL-415s were built and they’re not used on a continuous basis. As of this writing, we’ve only seen two of these water bombers in action from this vantage point.
According to the description of the video, this took place in Portugal and the crew was getting water from Rio Douro. You can actually see them pass over a dam before dipping down and getting to work.