let me guess: science was not your best subject in school?
You have not accounted for what it takes to hold the water in the right side at a level above the left side - you show it communicating at the bottom - that would be neccessary for the bottom run of the conveyor belt to pass from the low side to the high side.
If you or anyone could devise an energy-efficient way to pass the convery belt and is "air cube" cargo from the left side to the right side, you would be a mechanical genius. BUT: as to perpetual motion, you need to understand that the sum of all of the forces in your system are zero, there is no excess energy to harvest. In the missing piece of your puzzle (passing through the barrier from left to right) the pressure differential between the two sides that must overcome taking the cargo from left to right side would be exactly the amount of the buoyancy of the same cargo - the source of the force you think will drive the system.
Not only do you end up with zero net, but there loss of energy in the system would be the friction and force needed to move the sealing mechanism to seal the belt and cargo on its pass from left to right. In other words, to get any motion at all, you would need to add that amount of energy to the system to make it move (overcoming sealing mechanism losses, hyrdodynamic losses from moving through the water and mechanical losses from the flexing of the belt and the bearings of the two pulleys).
I applaud your generosity in offering your solution to perpetual motion freely to the rest of the world, but sadly, you have not stumbled upon something that is not understood, you just didn't understand it.