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Bridging the barrier between air and water is difficult for most animals, but for auks it is not a problem. First of all, when they dive they close a third eyelid, but it is transparent and actually allows them to see more clearly underwater. It's rather like wearing a contact lens. Once underwater, it's just a matter of flying. They don't use their legs at all, but because water is so much denser than air they have to put a lot of effort into flying and build up very big pectoral muscles. The penalty of that is that they are very heavy and actually quite clumsy fliers in air. They look more at home underwater where they pursue fish and sand eels, or escape if threatened from above. It's almost like they are becoming penguins, which are, of course, totally flightless.