Abstract Due to marine dynamics, sensor nodes of Underwater Wireless Sensor Networks (UWSN) are always in a state of constant motion. Together with low propagation speed of acoustic signal, nodes mobility brings challenge to UWSN MAC. A motion model of underwater node is established to solve the multiple access problems for underwater mobile networks. Based on AR mobility prediction algorithm, the negative effect of space-time uncertainty can be reduced and the probability that sending information arrives in the reservation slot can be improved. The simulation results show that AR(5) algorithm can decrease 74.8% of delay detection error. A new contention-based MAC protocol: P-MAC is proposed. NS-2 simulation results show that P-MAC can get a 10% -15% higher Packet Receptipn Rate (PRR) under the circumstance of wave motion.