It would depend on the water itself. Just because water is in a lake does not mean it is good or suitable for plants. The lake I live on is so acidic that I ended up with three different hulls for one boat over five years due to the gelcoat blistering from the high acidity level of the lake water. My lake water, or lake water like it, would not be a good choice for watering plants.Why would a natural water source pose a problem? It's not like plants are going to get disentary or parasitic diseases. There's really nothing harmful to kill... boiling water beforehand is just going to make the water harder.