Due to the complexity of flow structure in underwater explosion, there has been a difficulty in predicting structural response near a detonation origin. Many researches have been conducted to understand compressible and multi-phase flow in underwater explosion. For compressible flow governed by the Euler equation, solving the Riemann problem is essential for numerical simulation. In this study, we examine shock propagation in underwater explosion using an in-house code. The HLLC (Harten-Lax-van Leer-Contact) scheme is used to deal with the Riemann problem, and various boundary condition are used for implementing a reflected shock wave. With consideration of the infinite depth of water, sea bed, and free surface, we investigate how a shock wave is propagated and reflected in different boundary conditions.