The design of Medium Access Control (MAC) protocols for UWSNs poses many challenges because of their long propagation delay, high mobility, limited bandwidth, and high bit error rate. Due to these unique acoustic channel characteristics, most contention-based MAC protocols are costly. Thus, collisions and retransmissions should be efficiently handled at the MAC layer in order to reduce the energy cost and to improve throughput and fairness across the network. As a consequence, they do not perform as efficiently as their achieved performance in terrestrial networks. In this paper, we evaluate the performance of three recently reported distributed collision-free MAC protocols; namely, ED-MAC, DL-MAC, and GC-MAC under various operational conditions.