|Michael Mehta & Nelson Quiroga going dinghy diving- photo by Junie Quiroga|
|Nelson in full SCUBA gear - photo by Michael Mehta|
An important distinction needs to be made here between the water molecule H2O which contains chemically bound hydrogen and oxygen atoms that can't be easily separated except by electrolysis, and oxygen gas from the atmosphere that has dissolved into the water. Oxygen gas from the atmosphere is added to the ocean water by the mixing action of wind and waves and, as a by-product of photosynthesis by the plankton. Conversely, oxygen gas is depleted from the ocean by the fish who extract it using their gills.
Unfortunately it's not that simple. First off fish are cold blooded which means they have a much lower metabolism than us warm blooded humans, and that in turn means we need a lot more oxygen. With the average diver needing to breathe 1 litre of oxygen per minute, and the dissolved oxygen count of ocean water being only 7 ppm this works out to 192 litres of seawater a minute that would have to pass through any human operated gills in order to get out the necessary amount of oxygen. (A litre of seawater weighs approximately 1 kg. and a litre of oxygen weighs approximately 1.4 grams if you want to do the math yourself)
Another way of looking at this would be like trying to swallow a 45 gallon barrel of seawater every minute which is not only very impractical it's also not very efficient. An Israeli company called Like-A-Fish has tried to develop a more efficient method with a device that uses a high speed centrifuge to lower the pressure of seawater and let the oxygen escape into a storage bag for a diver to breathe. This process however, requires considerable battery power to function and a lot of complicated equipment so for now it's more practical to stick with the proven technology of standard SCUBA gear.