  We study ``draining bathtub'' as an acoustic analogue of a three-dimensional
rotating black hole. Rotating fluid near the sonic horizon necessarily gives
rise to the superradiant modes, which are partially responsible for the
thermodynamic quantities in this rotating vortex-like hole. Using the recently
suggested thin-layer method overcoming some difficulties from the well-known
brick-wall method, we explicitly calculate the free energy of the system by
treating the superradiance carefully and obtain the desirable entropy formula.
