We investigate the possibility of using orthogonal frequency division multiple access (OFDMA) for design of efficient underwater acoustic (UWA) networks. The major challenge for the OFDMA-based UWA communication systems is the Doppler-induced frequency scaling, which may cause severe intercarrier interference (ICI), and depending on the particular user subcarrier assignment, can introduce a destructive level of interuser interference (IUI). The commonly-adopted Doppler mitigation technique of OFDM based transmissions uses a single-resampling stage, which attempts to compensate for the common Doppler scaling factor shared by all propagation paths. While effective in peer-to-peer communication scenarios, this method might be suboptimal and may fail in a multiuser environment, where users can experience significantly different Doppler scaling effects. In this paper, we develop a unique OFDMA receiver front end, which enables compensation of each user-specific Doppler by means of multiple resampling branches. With this new design, we investigate two subcarrier allocation schemes, namely the contiguous carrier assignment, and the interleaved carrier assignment schemes. We demonstrate the effectiveness of multiple-resampling designs and study the differences between the two assignment schemes both via simulations as well as emulations obtained by real data recorded in the recent 2010 Mobile Acoustic Communications Experiment (MACE 10).