;D I think this will work
I have seen a similar idea but i dont think it was on ou...it had the same basic concept but it was rotated 45 degrees clockwise and there were pendulum like weights attached-the weights would lock and flip like many of the gravity wheels show but the boyancy acted as an upward force pushing on the weights then gravity would pull them down......The only problem i have found is with the tube as making an air tight seel for something you want to keep in motion is apparently very difficult. Best i could think of is something like a revolving door-but i haven't found a way to incorporate into the system-also the leak caused by the pully would still need to be dealt with.
Hope this helped in some way or another........aparodox