Ionic substances, from its name, form ions. When it is dissolved in water, the ions (cations and anions) are formed. For example, dissolving sodium chloride (NaCl), an ionic salt in water, it produces sodium and chloride ions (Na+ and Cl-). As the ionic solute is placed in the water, the water molecules break the ionic bonds as they surround the ionic compound.
On the other hand, polar covalent compounds don't form ions when they are dissolved in water. Instead they dissolved into molecules. There are no changes in the overall structure of the substance as compared with the ionic compounds. The water molecules are attracted to the polar side of the compounds but not strong enough to break the bond to form ions.