Is enthalpy of hydration always negative?
The definition of enthalpy change of hydration is the enthalpy change when one mole of aqueous ions are formed from one mole of ions in the gaseous state. All that happens during this process is that bonds/attractions are formed between the ions and water molecules. There are no bonds to be broken, and so there is no endothermic step in hydration, only the release of energy when the bonds between the ions and the water are formed.
Remember - bond breaking requires energy (endothermic step). Bond making releases energy (exothermic step).