Water is one of the three vital components any plant needs to survive. When mixing additives like salt or sugar into water it can alter the plant's ability to grow.
Saltwater is extremely detrimental to most plants and can seriously inhibit growth. Saltwater contains high levels of sodium that create a toxic environment for plant systems. Salt can also absorb water from plant roots causing the plant to wither and die. High concentrations of salt in soil will prevent the plant from gaining access to hydration, a necessity to survive and grow. Salt may even linger in the soil, preventing new plants from growing.
Sugar, like salt, can draw moisture away from plant roots, essentially choking plants to death. While you may notice the soil around the plant roots stays wet due to the sugar water, the ability to absorb that moisture is blocked.
Tap water is one of the best options when watering your plants. It is relatively free of harmful chemicals and additives. This allows the water to flow freely to the roots of the plant for absorption, ensuring healthy growth.