lol. after a bit of painfull thinking, i think that the water level would rise, since ALL the area inside the boat (the part under the surface level of the water) was taking up space before, but the bouyancy of the boat reduced the effect of the rock taking up some of the space. When the rock is thrown overboard, the majority of the boat will not move (if it is light wood or is inflatable or is fairly lightweight) whereas the rock will now be able to sink and occupy more of the space previously occupied by the water than it did when in the boat, due to the boats buoyancy. BUT none as this is definite, as it could depend on the mass of the rock, the mass of the boat, the volume of water, how much you weigh and even how warm it is. My advice is not to play with rocks in boats, or at least not to try and work out the physics.