What does it mean if you are coming up for air?
Coming up for air means to take a breather, to have a rest, or to take a break.
Fish have gills, so they do not need to come to the surface of the water to breathe. But many animals that live in and near water have lungs and they must come to the surface of water to breathe.
Therefore, when animals are coming up for air, they are taking a breath.

Terrapin
When humans are coming up for air – and not when they are swimming – it is a metaphor to get oxygen, to take a breath, to get a rest or a break. The rest can be to stop what you are doing for awhile, or to go for a walk, or do something different from what you were doing – and then come back to what you were originally doing.

Australian Sea Lion
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM