We present the implementation of new GNU Radio blocks that support mobile underwater acoustic communications. More specifically, to address the Doppler shift that occurs during the transmission of data frames at a very low data rate. We aim at long distance communications, which require low frequency and extremely narrow bandwidth modulation, and implies weak signals. We build upon our previous works on ad hoc underwater wireless communications, to handle constant or variable (linearly and non-linearly) Doppler shift patterns. Experimental results are discussed using simulation of underwater autonomous vehicles and underwater wireless sensors. Our main contributions are in the design of the decoder, implemented using the GNU Radio development toolkit.