I went sailing last summer and was thinking about how to explain the difference in buoyancy to surface tension. To a person they could seem like similar things. However, they are not. For buoyancy there is an upward force on the object from the water. The boat is buoyant and is maintained floating because of this upward force from the particles on the bottom of the boat. With surface tension there is only the particles on the surface of the water that affect the thing floating. Since this has less force mostly small things like paper clips can float on the surface of the water and be maintained solely from the surface tension of the water. Adding soap this surface tension and the small bonds maintaining the paperclip are broken and the paperclip sinks. Here is a similar explanation from science blogs.
