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I'd say that unihemispheric sleep and adaptations like it really are sleep - the brain activity on one side of the brain gives a characteristic sleep pattern. It certainly must satisfy the needs of an aquatic mammal like a dolphin or a whale since they have to be partially conscious to breathe by surfacing regularly. It does seem to affect the brain ...

Yes, according to calculations presented here and here the conclusion is that: The O2 diffusion coefficient in saturated air (15% oxygen) is 5,700 to 10,800 times greater than in water (60°C and 20°C respectively). And here is a paper using this difference to investigate the transfer of O2 through the tracheal system of a click beetle.