Human divers can feel a squeeze as they dive deeper, but once they equalize, they don't really feel the depth. Oxygen gets toxic when it's too concentrated (gasses compress at depths and this would concentrate any oxygen a mammal brings down). I assume aquatic mammals would die if they went down too far, much like a human divers would. Can they tell when they've gone too far, or does it just not happen because they would run out of air before they got down there? Can they get decompression sickness?