When underwater, a swimmer needs to hold on to any oxygen they can inhale, but must resurface after a short time to breathe again. Without resurfacing, an excess of carbon dioxide will poison the blood with carbon. That and/or a combination of water entering the lungs can lead to drowning.
Holding breath will be visible in images as a person's mouth being tight-lipped and cheeks puffed up. This tag should be used for humans who are underwater, not so much for fantasy creatures like mermaids.
