Coherence of broadband acoustic signals in very shallow-water regions is examined. To measure the frequency-dependent spatial and temporal variability of sound propagation in coastal regions, experiments were conducted using sound impulses in the form of m sequences centered from 0.6 to 22 kHz. In these experiments the water depth was approximately 15 m. The receiver hydrophones, with spacing less than a few meters, were located in a range of 200--800 m from the source. High-resolution oceanographic data taken simultaneously with the acoustic data were used to analyze the broadband propagation. It is shown that the temporal coherence changes significantly as a function of frequency and that the nature of this variation depends on location and time, since the ocean environment (e.g., the sound-speed variability, current, and the surface/bottom boundary conditions) is location and time dependent. However, for hydrophone spacings of a few meters, no significant spatial decorrelation of the signal has been observed.