The amount of water in your hookah base is essential for a good smoking experience. You should keep the downstem 1 to 1,5 inches deep in water. Depending on your hookah base's volume and the size of your hookah's shaft and downstem, the amount of water amount might vary. Therefore, for smaller models, you might need to fill less water.
If you notice that the draw of your hookah is tougher than usual, it could be due to an excess amount of water. Another sign that you poured too much water is if you start to see water appear in your hose while smoking. This won't help for a smooth session, so reducing the amount of water in the vase is crucial.
Also, some manufacturers place a level mark on the base, indicating how much water to pour in for a better experience.
If you want to learn how to pack a hookah bowl, check this article.