Conductivity is the inverse of resistivity. The resistance of a material is given by R = V/I, or the voltage V that should be applied across the material to create a current I that flows through it. Resistance is measured in Ohms.
Conductivity of a material is given by I/V, or it is the current I that flows through the material when a voltage V is applied across it. Conductivity is measured in Siemens.
The conductivity of water depends on its purity and the type of impurities dissolved in it. The conductivity of ocean water is approximately 53 mS/cm. City water which does not too many dissolved impuroties has a conductivity of 50 µS/cm.
Pure water has a conductivity of .055 µS/cm