Problem for terrestrial is to keep respiratory surfaces wet, avoid water loss for animal. Multiple convergent evolutions. Gill bailers to create current over gills. Pump through mouth. Not like crocs.
Mammals have suction lungs.
Under control of hindbrain and sensitive to blood CO2, coordinate with heart control. Some have air sacs. Diving beetles carry air bubble. Water vascular system of echinoderms. Internal canals formed from coleom plus tube feet.