Crabs have gills under their shell near their legs. They have an appendage that pushes water over them called a scaphognathite and can seal the water inside the shell with articulating plates to keep the gills wet. When they are out of the water and blowing bubbles out of what looks like their mouth, that's the result of them breathing with their gills. As long as they can keep finding more oxygenated water, aquatic crabs can stay out of the water for a surpassing long time, like a day or two. Land crabs have a weird lung gills hybrid organ but they're adapted to land life and just seek out water to keep that organ wet. Coconut crabs are a type of hermit crab that can't swim and will drown in about an hour if they are swept away or fall into water