What animal lives in and out of water?

they have gills not lungs

Some species have both. Betta fish, for example, have an organ called the "Labyrinth, that allow them to breathe air. Snakeheads can also breathe air, which led to the belief that they can live on land for three days. Other fish can breathe air as well. It allows this fish to live in water with lower oxygen supply, where many fish would drown. (I know, it's weird to talk about fish drowning...)