In this paper, the authors explain the Medium Access Control (MAC) problem in underwater wireless acoustic networks. They explore the random access and handshaking (i.e., RTS/CTS) techniques in both single-channel and multi-channel network scenarios. They model and analyze these two approaches, and conduct extensive simulations to study their performance in various network conditions. Based on their results, they observe that the performance of both approaches are affected by many factors such as data rate, propagation delay and packet size. Their results show that the RTS/CTS approach is more suitable for dense networks with high date rate, whereas the random access approach is preferred in sparse networks with low data rate.