  As the dominant information carrier in water, acoustic wave is widely used
for underwater detection, communication and imaging. Even though underwater
acoustic communication has been greatly improved in the past decades, it still
suffers from the slow transmission speed and low information capacity. The
recently developed acoustic orbital angular momentum (OAM) multiplexing
communication promises a high efficiency, large capacity and fast transmission
speed for acoustic communication. However, the current works on OAM
multiplexing communication mainly appears in airborne acoustics. The
application of acoustic OAM for underwater communication remains to be further
explored and studied. In this paper, an impedance matching pentamode
demultiplexing metasurface is designed to realize multiplexing and
demultiplexing in underwater acoustic communication. The impedance matching of
the metasurface ensures high transmission of the transmitted information. The
information encoded into two different OAM beams as two independent channels is
numerically demonstrated by realizing real-time picture transfer. The
simulation shows the effectiveness of the system for underwater acoustic
multiplexing communication. This work paves the way for experimental
demonstration and practical application of OAM multiplexing for underwater
acoustic communication
