One way to find this would be to take the distance above sea level of one and subtract the distance above sea level of the other.
So with this you'd get 1 above sea level minus negative eight above sea level (which is 8 under sea level), so 1 - (-8) which is 1 + 8, which is 9.
However, the easier way to do this specific is to draw a line for sea level, draw a short line going above it to represent 1 meter, and draw a longer line under it to represent 8 meters, then find the difference between the diver and the captain by adding up the lengths
