There are many peculiarities in underwater acoustic channel as comparing with radio communication channels, including random spatial-temporal-frequency variability, large attenuation, severe multipath effects, strict bandwidth limitation, and high noise levels. Therefore some present advanced signal processing systems, or even some basic principles, such as Shannon theorem, in radio communications cannot perfectly be employed in underwater acoustic communication. Therefore, exploring some innovative signal processing systems that can be suitably employed in peculiar underwater acoustic communication channels is a valuable and challenging research topic. Sound transmission laws in the sea including transmission loss, multipath effects, fluctuations of transmitted sound, and marine noise, especially their impacts on digital underwater acoustic communications and the possible countermeasures to adapt to them are discussed in detail in this chapter. They are the physical basis for designing the signal processing systems employed adaptively in underwater acoustic communication channels, which is also a basic premise for establishing new digital underwater acoustic communication equipment.