Skin-diving is swimming under the water with only light breathing gear — a mask, a snorkel, sometimes a small air tank — but without a thick protective suit. You feel the water on your skin, which is how the name arose. People do it to look at fish and reefs in warm, clear water. Though actually, in real talk the line is messy: many people now just say snorkeling if they stay near the top with a tube, or scuba if they carry tanks. Skin-diving is the older sport name for going under with light gear and no wetsuit. It is not holding your breath with no air at all (free-diving), and it is not heavy diving in a full suit.