After 1 hour, the diving bell was -1/2 mile relative to sea level.
Since the rate of decent is constant, we can use two ratios to represent the depth at 2 different times. To make things easier to write, I'll use decimals to represent time and depth.
-.75 / 1.5 = x / 1
Now solve for x
-0.5 = x
So after 1 hour, the diving bell was -1/2 mile relative to sea level.