Convert to eighths:
1/2=4/8
1/4=2/8
Now we have everything in eighths: -4/8, -2/8, -5/8, -7/8.
The surface of the water is 0, and all the treasure is underwater. The deepest is the biggest negative number: -7/8, where the copper coin is. On a number line we have zero at the right hand end of the line. To the left of zero we have eight subdivisions and the number -1. We can label divisions to the left of zero at -2/8, -4/8, -5/8, -7/8 that is, 2, 4, 5, 7 small divisions to the left of zero. The leftmost is where the copper coin is. Label this division copper, then necklace at 5 divisions to the left of zeros, then gold at 4, then bronze at 2.