Recent simulations and experiments have shown that the viscous-range temperature spectrum in water can be well described by the Kraichnan spectral model. Motivated by this, a tractable expression is developed for the underwater temperature spectrum that is consistent with both the Obukhov-Corrsin law in the inertial range and the Kraichnan model in the viscous range. In analogy with the temperature spectrum, the formula for the salinity spectrum and the temperature-salinity co-spectrum are also derived. The linear combination of these three scalar spectra constitutes a new form of the refractivity spectrum. This spectral model predicts a much stronger optical scintillation than the previous model.