We describe the application of the implicit particle filter to a shallow water model of nearshore circulation. This is a model with approximately 30,000 state variables. We assimilate gridded observations of the two horizontal velocity components at each of 16 locations. Results are shown for two distinct flow regimes, one characterized by regular shear waves, and the other a more complex aperiodic flow that we show is qualitatively different. The system runs efficiently on a single workstation.