Bubble columns replenishes the player's air supply. However, the player's mining speed is still reduced as if the player's head is submerged in normal water.

How to reproduce

1. Place down a magma block underwater, creating a bubble column
2. Stand inside the bubble column in Survival mode, notice how the air supply replenishes
3. Place a block (e.g. snow block), and break it with the proper tool (e.g. iron shovel, which normally breaks snow blocks instantly on land)
4. Notice how the block breaks much slower than on land (in the screenshot's case, not instantly)