The present study investigates the velocity fields of plunging breaking waves impinging on a three-dimensional simplified ship-shape structure in a laboratory wave tank. Green water was generated as the waves break and overtop the structure. Bubble image velocimetry (BIV) was used to measure the velocity field of green water along the centerline of the deck. Two plunging wave conditions were tested and compared: one with waves impinging on the vertical wall of the structure at the initial still water level; the other with waves impacting on the horizontal deck surface. The velocity fields are quite different for the two cases even though the incoming wave heights and the wave periods are nearly identical. It was observed that the maximum horizontal velocity is higher for the case with waves compacting on the deck. The waves also passed the deck quicker than the other case. For both cases the profiles of the green water velocity shows a non-linear distribution with the maximum velocity occurring near the front of the water.