It depends... First, where are you standing in the boat? If its near the stern or near the railing at the end, you may end up in the water. Second, is the boat accelerating or at a constant velocity? If the boat is accelerating, it will continue to accelerate while you will not (you will actually begin to decelerate due to air resistance), and again, if you are near the stern (close, close, close) your chances of landing in the water are much more likely. Next, you would have to jump fairly high... not too high, but high enough to clear objects such as the railing, etc. This will also give you more time in the air, away from the accelerating boat, and thus more time to decelerate, making the chances of landing in the water much higher. The only difference is that if the boat is at a constant velocity, you will not so much have to worry about it accelerating away from under you, but just from your deceleration due to the air resistance. Hope that helps to answer your question!