That is because your body has mass. The mass of an object does not change in space. Everything in the world is made up of matter. Matter also has mass. Mass is the measure of the amount of matter in an object.
Weight is a measure of the force of gravity on an object.
Mark the level with a marker. Carefully place rocks, or other objects that will sink, into the water, one at a time. Continue doing this until the container overflows. Try to help them understand the principle that matter takes up space, and cannot occupy the same space as other matter. Use a balance for measuring the mass of an object.