Salt indirectly affects buoyancy but it is not required.. Buoyancy is the force of the weight of an object minus the weight of the water it displaces. Pure fresh water is lighter than, water that has solutes dissolved into it. So since salt water which has the solute salt dissolved into it weighs more than fresh water, objects will be more buoyant in salt water than in fresh water. But this applies to any solute, for example you can increase the buoyancy of an object in fresh water simply by adding sugar. So to answer your question, any solute that increases the weight of a given solution will increase the ability of object submerged in it to float, including but not limited to salt.
Yes. Buoyancy results from lighter and less dense materials rising while heavier and more dense solids, liquids or gases sink. Adding salt to water will increase the water's weight and density. That is why a person might be less likely to drown in the Dead Sea than other bodies of salt water.
Salt affects buoyancy of an object. This is because if an object is immersed in salt water, it will have high buoyancy than the one that is immersed in fresh water. This is because salinity levels in water increases buoyancy levels. Read http://www.ehow.com/how-does_4564443_boats-float.html.