Why the potential energy is unlimited?

[Zhang Yalin]

It’s easy to let the air out because the atmospheric pressure inside and outside of the balloon is the same.

[TinselKoala]

That's simply wrong. The weight of the water in the column "pulls a vacuum" and the water wants to fall until the level inside the tube is the same as the level in the pan. The sealed tube prevents this. As soon as you put the balloon in there, it is the same as putting a bubble, or opening a valve at the top of the tube for a moment. The water level drops by the same volume as the air in the balloon. Now, to get that air out of the balloon, you have to RAISE THE WATER BACK UP. No matter how you do it.

Don't believe me? Get a water glass, a pan of water, and a piece of tubing. Set up the full glass column and pan with water as in your diagram. Now introduce one end of the tubing into the column, leaving the other end open to the air. What happens? How do you get the water back up into the glass?

[TinselKoala]

Perhaps this little video will help with understanding:
.
.
http://www.youtube.com/watch?v=rPEOPWG_gh8

[Zhang Yalin]

No need to make a hole in the device to release the air.

[TinselKoala]

OK, Yalin, it is now up to you.
Please explain how to release the air from the balloon without raising an equal volume of water back up into the column.
Or, if you simply withdraw the balloon while full, how do you do this without raising up that volume of water.
Or, if you have an opening into the balloon as shown in your first diagram, how do you keep the balloon from expanding from the suction, just as in my video above...and how could your "splints" possibly deflate the balloon without also pulling up an equal volume of water which weighs much more than your balloon...