Why do turtles live underwater when they breathe air?

Turtles do not live permanently underwater; they are semi-aquatic reptiles. Most turtle species must surface regularly to breathe air through their lungs, using their nostrils or mouths on land, just like humans. Their shells afford protection, not respiration.

Some fully aquatic turtle species such as the snappers (Chelydra serpentina), the matamata(Chelus fimbriata), and a few others can take in oxygen through specialized mouth or cloacal linings from water, even though they mostly breathe air like other turtles from their lungs by periodically surfacing. While most of their oxygen exchange occurs through skin tissues exposed to or lined with water rather than air, those aquatic or amphibious specimens still respire and must take in their own supply of fresh air regularly, unlike true water-breathing lifeforms or fish for instance.