I call bulls***. Seawater is typically less than 10 ppm for oxygen content - if you're going to match the normal proportion of the atmosphere and feed a diver 21% oxygen by volume (go below 16.5% and you'll black out), then for a normal tidal breath of 500 ml, it'll have to contain 105 ml (at 1 atm) of oxygen per breath. Typical resting respiratory rate for adults is maybe 15 breaths per minute, so you have to come up with 1575 ml of oxygen per minute.
So, bottom line, at 10 ppm you'll need to process 157,500,000 ml of seawater per minute, or 157,500 liters per minute. Now, a fairly high-powered shop vac will suck in 125 cubic feet per minute, or about 3500 liters per minute, so imagine something that sucks in water 45 times faster than a shop vac, take that something and strap it to your back, and that's what we're talking about. And this is a best-case scenario - the minute you have to move or exert yourself or anything, your respiratory rate and oxygen requirements are only going to increase. Hope whatever this thing is, it comes with a hell of an extension cord ;)
I like the general idea of this general_re calculation, but a similar calculation would seem to show that large fish (e.g., large sharks) could not exist (when it uses its muscles, it would need too much oxygen).
This post contains all the things I love about the posts on FreeRepublic.
Nice, concise and humorous.
Plus all of us who received woefully inadequate public school educations can grasp the science involved.
Thanks