The deck wetness and the impulsive water pressures induced by shipping water on the deck are serious problems of a ship operating in rough seas. For these problems, model experiments were conducted in extreme head seas in a towing tank. In this study, relations between the probability of deck wetness and the following matters are discussed : 1. significant wave height 2. mean wave period 3. ship speed 4. significant values of ship motions 5. relative bow motion. Relations between the peak values of impulsive pressures, relative bow motion and impact velocity of shipping water are also discussed. Moreover, the prediction of the probability of deck wetness was made, applying the empirical expression of the dynamic swell-up proposed by R. Tasaki, and compared with the experimental results. Finally some recorded patterns of impulsive water pressures are discussed in detail.