  Investigating wave propagation in fluid enables a variety of important
applications in underwater communications, object detections and unmanned robot
control. Conventionally, momentum and spin reveal fundamental physical
properties about propagating waves. Yet, vast previous research focused on the
orbital angular momentum of acoustics without thinking about the existence
possibility of spin due to the longitudinal wave nature. Here, we show that
underwater acoustic wave processes the non-trivial spin angular momentum
intrinsically, which is associated with its special spin-orbital coupling
relation for longitudinal waves. Furthermore, we demonstrate that this
intrinsic spin, although unobservable in plane wave form, can be detected by
four approaches: wave interference, Gaussian exponential decay form, boundary
evanescent wave, and waveguide mode. We further show that the strong
spin-orbital coupling can be exploited to achieve unidirectional excitation and
backscattering immune transport. We hope the present results can improve the
geometric and topological understanding about underwater acoustic wave and pave
the way on the spin-related underwater applications.
