Driven by the huge demand to explore natural resources, conducting ocean research, and warning from natural disasters, underwater acoustic communications have become a fast-growing field in the last decades. Due to the underwater acoustic channel (UWAC) characteristics, such as multipath effect, severe path loss, high spread and shift Doppler, and long delay spread, communication, and localization become challenging tasks. This paper proposes a novel technique based on chirp spread spectrum (CSS) modulation to communicate and localize underwater acoustic channels. This technique uses differential encoding, making it more resilient to Doppler shift. It uses the orthogonality of chirps to generate the payload, which makes it more resilient to inter-symbol interference (ISI). Moreover, channel estimation and equalization processes are introduced to demodulate the signal, and Doppler estimation and compensation processes are added to minimize the Doppler spread effect. Finally, the synchronization process is introduced to fix the start time in communication and also used to calculate the time difference of arrival (TDOA) between receivers for the localization.