Somewhat surprisingly, fire and moving water can pose similar photographic challenges. With both fire and water, you may want to get a great deal of depth of field to get different parts of a photo in focus. High depth of field images tend to have the appearance of sharpness because nothing is out of focus.
To get high depth of field, you need to close the camera down and use a small aperture on your lens. This implies a long exposure, even considering the brightness of flames and the lightness of foam in a waterfall.
But with a long exposure, both flames and moving water can appear almost solid, very different from the way fire and water look to our naked eyes.