In a newly created world in the 1.7 snapshot, I've started seeing natural pockets of air in large bodies of water. Usually, there are 2-4 air-pockets grouped together, and they're normally about 2x2x3 in size.

One of the screenshots (2013-10-23_21.05.34.png) shows a clear view of a three pockets with the help of a nightvision potion (otherwise they would be really hard to make out.)

EDIT:
I figured I could provide an example seed for you: 5934833244926967597
/tp 1952 64 -3941