The compaction of a granular assembly immerged in a fluid implies complex interactions between viscous forces and grain contact forces. We present the study of the compaction under tapping of a granular packing fully immerged in water. In order to follow the interaction between the grains and the fluid, we measured the pore pressure at the base, related with suspended particles in the packing. A simple model is proposed to understand the time relaxation of the pore pressure.