1. An apparatus for placement on or in a body of water for hyperspectral imaging of material in the water comprising an artificial light source and a hyperspectral imager which are arranged so that in use light exits the apparatus beneath the surface of the water and is reflected by said material before re-entering the apparatus beneath the surface of the water and entering said hyperspectral imager, wherein said hyperspectral imager is adapted to produce hyperspectral image data having at least two spatial dimensions, and wherein the apparatus is arranged to estimate a spectral attenuation coefficient of ambient water and to adjust a spectral output of the artificial light source so that a predetermined light spectrum arrives at said material.