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In this paper, we investigate the multichannel MAC problem in underwater acoustic sensor networks. To reduce hardware cost, only one acoustic transceiver is often preferred on every node. In a single-transceiver multichannel long-delay underwater network, new hidden terminal problems, namely, multichannel hidden terminal and long-delay hidden terminal (together with the traditional multihop hidden terminal problem, we refer to them as "triple hidden terminal problems”), are identified and studied in this paper. Based on our findings, we propose a new MAC protocol, called CUMAC, for long-delay multichannel underwater sensor networks. CUMAC utilizes the cooperation of neighboring nodes for collision detection, and a simple tone device is designed for distributed collision notification, providing better system efficiency while keeping overall cost low. Analytical and simulation results show that CUMAC can greatly improve the system throughput and energy efficiency by effectively solving the complicated triple hidden terminal problems.