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The acoustic communication channel is frequency selective with long memory, leading to severe intersymbol interference (ISI). To mitigate ISI, equalizer becomes an indispensable module in the receiver structure. However, the time-varying nature of the underwater acoustic environment imposes unique challenges to the design of an effective equalizer. First, the equalization process needs to be performed on a block basis and the block length could be short. Second, concerning that the dynamic acoustic medium makes the newly acquired channel information readily outdated, it is desirable that the equalizer performance is robust against the inaccuracy of the channel information when the transmission scheme involves cross-block reference. In this paper, we consider a statistical semiblind equalizer implemented by the Gibbs sampler techniques. The proposed equalizer con- ducts channel estimation and symbol detection in a joint manner, and it is robust to the accuracy of the channel information. The effectiveness of the proposed semiblind equalizer is demonstrated using both simulated and the 2008 Surface Processes and Communications Experiment (SPACE08, Martha's Vineyard, MA) in-water experimentation examples.