What do I have to do to figure out " What % of an object's mass is above the water line if the object's density is 0.82g/ml " with just that information?

18%

Explanation:

There are two equal and opposite forces on a floating object: weight and buoyancy.

W = B

The weight of an object is its mass times gravity: W = mg

Buoyancy is the weight of the displaced fluid: W = mf g

Plugging in:

mg = mf g

m = mf

Mass is density times volume:

ρV = ρf Vf

Solving for the ratio of Vf / V:

Vf / V = ρ / ρf

Given that ρ = 0.82 g/mL and ρf = 1.00 g/mL:

Vf / V = 0.82

That means 82% of the object's volume (and therefore, 82% of its mass, assuming uniform density) is submerged. Which means that 18% is above the water line.