Experimental measurements of signal coherence and array signal gain are reviewed for both deep and shallow water sound channels. The signal gain is related to single path or modal coherence lengths through well-known relationships in the statistical theory of antennas. Signal gain measurements in the transverse and longitudinal directions are proffered as the optimum measures of coherence lengths for both broadband and narrow-band signals. Using this technique measurements (<1 kHz) are presented that show for the deep water cases lengths on the order of 300 wavelengths can be achieved while in the downward refraction conditions of a shallow water waveguide lengths between 30 and 100 wavelengths are realized. The measurement of broadband and narrow-band coherence and correlation functions are discussed with emphasis on the role of partly coherent noise backgrounds and multipath interference effects as well as averaging constraints. These results are interpreted with coherence models based on sound scattering from the volume and boundaries of the waveguide. The requirements for the numerical modeling of the signal coherence are presented.