Do not use just water. Ethylene glycol coolant protects against freezing in cold weather and boiling over in hot weather, as well as providing corrosion protection for your engine. Using the wrong type of coolant can damage your engine.
Before adding any coolant, check your owner's manual for the most precise and up-to-date recommendations for your specific vehicle. It may also specify a particular color of coolant recommended by Jeep. If you're unsure, consulting a Jeep specialist or a qualified mechanic is always a good idea.