can any one help me with a renewable energy question i have.

hi i am a practical thinker but i have no maths skill what so ever. I need to find out the energy that could be be gathered from a a 1000kg object falling in water to 100m in depth . Would be more energy than it would take to float it back to the surface using air. the air would of course have to be compressed so that it could be pumped 100m under water.
very strange question with a lot of variables i know but if some one could give me a answer i would be very grateful.
the question relates to a renewable energy source i'm trying to design.
any help would be much appreciated