I think it is just below the ocean? If the world was made before the update the world will not add one in, and then you will need to either beat the game and craft a shimmer bucket or just cheat some in.
If the world was generated before you got the 1.4.4 update, the Shimmer pool won't appear. That would explain it.
World generation changes generally won't carry over between updates because of how the elements of a world are decided at world creation. It requires new worlds to get the updated world generation changes.