To determine how deep the swimmer's dive was, we need to identify the lowest point (the vertex) of the parabola on the graph, which represents the swimmer's vertical position.
From your description, the x-axis represents the distance from the pool's front wall, and the y-axis represents the vertical position. A negative value on the y-axis indicates that the swimmer is below the water's surface.
If the graph shows that the lowest point of the parabola is at a y-value of, for instance, -4, then the swimmer's dive would be 4 units deep.
Please check the specific y-value of the vertex of the parabola in the image you referred to, and you can conclude how deep the swimmer's dive was by taking the absolute value of that y-coordinate.