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In this paper, a robust detection scheme is proposed for high data rate single-carrier multiple-input-multiple-output (MIMO) underwater acoustic communications. The new scheme adopts turbo block decision-feedback equalization, where the soft-decision equalizer performs successive soft interference cancellation of both the intersymbol interference in the time domain, and the multiplexing interference in the space domain. Attributed to the inherent advantage of the block decision-feedback equalizer (BDFE) over other conventional equalizers, the MIMO turbo detection algorithm provides high performance with fast convergence speed. With the interblock interference properly removed, the MIMO BDFE is performed with overlapped information blocks without guard intervals, thus a high transmission efficiency is guaranteed and the performance degradation at the tail of each block is prevented. The MIMO channel is treated as time invariant over each small block, and is estimated with either the pilot symbols in the training mode or the previously detected symbols in the decision-directed mode. The proposed detection scheme has been tested by extensive experimental data and proved to be robust to different transmission environments. The experimental results for the undersea 2008 Surface Processes and Acoustic Communications Experiment (SPACE08) and the 2008 Gulf of Mexico Experiment (GOMEX08) are both reported.