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 the horizontal coherence length through relationships from the statistical theory of antennas. Signal gain measurements in the transverse direction are proffered as a practical measure of coherence length for both broadband and narrow-band signals. Using this technique, measurements at frequencies near 400 Hz are presented that show for the deep water cases, lengths on the order of 100 wavelengths can be achieved, while in the downward, refraction conditions of shallow water waveguides with sand-silt bottoms, lengths on the order of 30 wavelengths are realized. Recent measurements of broadband and narrow-band coherence and signal gain are discussed with emphasis on the role of partly coherent noise backgrounds due to coastal shipping, multipath interference effects, and averaging constraints. In particular, the results of an experiment conducted in the Straits of Korea will be presented where array gain and sonar images were obtained in the presence of a strong coastal current.