For marine seismic sources quite efficient conversion of P-waves to S-waves can occur at hard seafloors, e.g., carbonate horizons in tropical waters. The S-waves are reflected back from structures at depth and are reconverted to P-waves in the water before detection by the receiver array. Such PSSP reflections can carry useful information on the structure beneath the sea bed but are most significant at large offsets and so are not easily stacked with a conventional normal moveout (NMO) procedure based on a hyperbolic time trajectory.A two-layer stacking procedure that separates the water layer from the region below the seafloor provides a very effective means of extracting the PSSP arrivals, but also works well for P-waves. There is no direct analytic form for the stacking trajectories but they can be calculated quite efficiently numerically. A further advantage is that the stacking velocity for S-waves in the lower layer can be interpreted directly in terms of S-wave propagation, so that S-wave interval velocities can be found. Stacking procedures based on such simple physical models are likely to be useful in other cases where attention needs to be focused on a particular aspect of the wavefield.