[birdzoom (OP)]

Please have a look at my idea to use deep ocean water pressure to get energy.
It looks a bit like a perpetual motion machine.
The idea is that a ball would roll on down in a normal, open air, tunnel (TUNNEL A) and released in a tunnel (TUNNEL B)filled with water at the same pressure as the ocean at each level.
Tunnel B is just to make the ball come up straight and drive some mechanism to get energy. You could also get some energy out of the free fall.
So energy from the free fall down and from the lifting up.
I assume the balls would be slightly less dense than water at sea level.
Please have a look at my drawing:
http://i53.tinypic.com/353av7s.jpg [nofollow]

Birdzoom.

[SeanB]

Tunnel A would fill with at least the volume displaced by the ball each time it went down, and this would either have to be pumped out or the system would stop after the first ball leaves the valve and rises..

[MikeS]

Apart from water getting into tube A the ball would have to be heavy enough to open the valve against water pressure in which case it would not float. 

There is a good reason why perpetual motion machines do not exist it's called the second law of thermodynamics.  Energy is conserved (the first law of thermodynamics) but useful energy after being used to do work becomes low quality 'waste' energy. 

The potential energy of the ball at the top of tube A becomes kinetic energy as it accelerates down the tube.  That energy is dissipated (ultimately as heat) upon contact with the valve.  If the valve were to open then the surplus energy of the ball would be absorbed by the water.