Torches can be placed underwater when there is a block above them.

If you place a torch in water, normally it attaches briefly then breaks and the space is refilled with water. This is still the case, unless there is a block directly above the torch. Then the torch attaches to the wall and the space around it fills with air. If you break the torch the air remains.

Edit: Now that I think about it, this is probably related to MC-128690.