Graduated Neutral Density filter
When shooting with the sky in the frame, you may require the use of a graduated neutral density filter, which darkens the top half of the frame. They come in a variety of densities, depending on how much you need to darken the sky. They also come in different gradients, from soft transitions to hard. Seascapes often benefit from hard graduated filters (they transition from light to dark very quickly). So Tantse used a 0.75 hard ND grad to darken the sky.