Label the forces acting on the box.
(Please say where each force belongs)

The box first accelerates with a negative (downward) acceleration until it hits the water. Upon hitting the water, the box experiences a balance of forces (50 N downwards due to gravity and 50 N upwards due to the water). Thus, the box will finish its motion moving with a constant velocity.

Identifying which forces are acting on an object is pretty straightforward. You need to have an understanding of the different types of forces and how they affect objects, such as gravity pulling objects down or friction acting against an object's motion. Then, if it helps, you can draw a free-body diagram.

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.