This reminds me of a discussion, all right an argument, that I had as a young lad with my cousin. We were on a beach, watching some pedalos bobbing about and my Cousin maintained, very forcibly - he was bigger than me, that if the floats were filled with compressed air, they would be more buoyant than floats filled with air at atmospheric pressure. He reasoned that air was lighter than water so, therefore if you could get more air in to the floats, the lighter they would be. My argument, which cost me a bloody nose by the way, was that a partial vacuum in the floats would make then lighter.