Will an object 20ft below sea level have a different absolute value than an object 20ft above sea level ... explain

No, the objects below the sea and above both have the absolute value of 20 ft.

The absolute value is how far the number is away from zero - it doesn't matter if it is in the positive or negative direction

For example, if you drove 20 miles from your house in one direction while your friends drove 20 miles in the other direction - both of the distances of you and your friend from the your house is 20 miles.

Distance is a great example of the absolute value because it is always positive. It doesn't matter what direction, it is always 20 miles from your house.