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How to achieve high speed, large capacity and bandwidth-efficient digital communication in shallow water conditions is challenging work. Recently, Orthogonal frequency division multiplexing (OFDM) has received considerable attention for UWA communication. In this paper, the authors investigate time-varying channel impulse responses with different transceiver moving speeds. For this channel model, the performance of MMSE and LS channel estimation methods are evaluated for an OFDM UWA system, based on block-type pilot and comb-type pilot (combined with the linear interpolation method) separately. The results are based on considering three different modulation schemes: BPSK, QPSK and 16QAM. The work is fundamental to the design of an adaptive modulation and bit loading OFDM UWA communication system in a changing channel environment, aiming for achieving optimal performance.