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To support the retrieval of accurate and up-to-date information from beneath the world's oceans, it is desirable to deploy underwater sensor networks. However, since radio frequencies (RF) do not propagate well underwater, acoustic channels are employed to support long-distance and low-power communication in underwater sensor networks even though acoustic signals suffer from long propagation delay and have very limited bandwidth. In this paper, we first show the limitation of conventional MAC protocols in the proposed environment; and introduce a new MAC protocol: the adaptive propagation-delay-tolerant collision-avoidance protocol (APCAP). We demonstrate that, in channels with long propagation delay, APCAP significantly outperforms those MAC protocols that are devised for RF-based networks.