We can usually set the level at y=0. The sign when diving can be either positive or negative. As we are usually working below the sea level, it makes sense that a diving of 40 feet can be represented by y=40, but using negatives values is also possible.
Then, an elevation from 40 feet below sea level has a negative value if the diving is considered positive (y=40), or a positive if the diving is considered negative (y=-40)