The propagation behavior of high-frequency intensity-modulated signals through turbid water is of significant interest for underwater laser ranging, imaging, and communications. Prior experimental measurements have focused only on the magnitude response of the underwater optical channel to forward-scattered and unscattered modulated light. In this study we include, for the first time to our knowledge, both the magnitude and phase of the underwater optical channel to forward-scattered light. The magnitude and phase response is measured out to 1 GHz, using three different artificial scattering agents in scattering environments in excess of 25 attenuation lengths. The phase response provides additional insight into the behavior of forward-scattered light carrying high-frequency intensity modulation.