The boat would sink a little bit in the water and move in the direction that the melon was thrown to you.
To understand why the boat would sink a little bit in the water and move in the direction that the melon was thrown, we need to look at the concept of Newton's third law of motion and the principle of conservation of momentum.
According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When you throw the melon towards the boat, you exert a force on the melon in one direction. As a result, the melon exerts an equal and opposite force on you in the opposite direction. This force causes you to move backward slightly.
Now, let's consider the conservation of momentum. Momentum is the product of an object's mass and its velocity. According to the principle of conservation of momentum, the total momentum of an isolated system remains constant before and after an interaction.
In this case, the whole system consists of you, the boat, and the melon. When you throw the melon towards the boat, the melon gains momentum in one direction, while you and the boat gain an equal and opposite momentum in the opposite direction. This means that the boat and you move slightly in the direction opposite to that of the melon.
As a result of this momentum exchange, the boat would sink a little bit in the water. This sinking is due to the boat and you collectively gaining momentum, which requires some displacement or movement in order to conserve momentum.
It's important to note that the boat sinking and moving in the direction of the melon's throw would only happen if the forces involved are significant compared to the weight and momentum of the boat. In reality, the weight and momentum of the boat would likely be much greater than that of the melon, so the sinking and movement would be minimal and hard to observe in most cases.