Sodium and water have a very volatile chemical reaction. When they come in contact with each other they produce massive amounts of heat due to the chemical reaction. Heat is part of the fire triangle. Heat, oxygen and fuel are the three things needed for fire. Because the heat produced the fire becomes more intense instead of being put out. Water is usually used to put out fires because it absorbs the heat and removes the heat from the fuel. No heat no fire. But since a chemical reaction between sodium and water produces heat water isn't only useless against a sodium fire, water makes it worse.