A perfect absorber design is proposed here that uses a periodic array of pyramidal nanostructures, made from a natural hyperbolic material bismuth telluride (Bi2Te3) on a substrate. A numerical study is carried out to investigate the absorption of solar radiation and to determine the suitable ranges of the geometric parameters for the proposed structure submerged in water. The results show that the proposed structure can achieve absorptance values greater than 99.9% in the wavelength range of 300-2400 nm, which covers most of the solar radiation spectrum. The underlying mechanisms are attributed to the combination of the slow-light effect and the gradient index effect. Optical properties of this type of absorber are affected by the geometry of the nanostructure (such as the height and the top and bottom widths of the pyramid), the distance between two adjacent pyramids, as well as the material and thickness of the substrate. Nevertheless, extremely high absorptance can be achieved even with some variations of these parameters. This study could open a route for effectively harvesting solar energy in photothermal conversion processes in water.