Surface tension of water
The hypothesis that adding salt to water and increasing the water temperature will reduce its surface tension is correct.
Insects such as the water strider are able to walk on water without sinking. Their legs distribute their weight on the surface of the water so so as to ensure that the downward force applied onto the water surface is not sufficiently large enough for the insect to sink into the water. Similarly, even though paper clips and needles have a higher density than water, they can be made to float on the water surface without sinking because of surface tension.
What would happen if the science fair project was repeated with other liquids such as oil or alcohol?
The experiment can be also repeated by adding other substances like sugar or milk into the water.
Surface tension - http://en.wikipedia.org/wiki/Surface_tension
Surface tension - http://physics.about.com/od/physicsexperiments/a/surfacetension.htm