Well, it is important to note that the measurements for absorption and transmittance are inversely related, a low absorption means the substance was not as effective in scattering and or absorbing the specific wavelength of light, I assume, and that most of the wavelength of light has passed through the substance and or has been transmitted, thus it would have a high transmittance.
The substance will not sink in water, as it Dencity is (0.5)is less than the density of water (1gcm).