Electolysis of H2O to produce O2(g), in a small, portable unit Full disclosure, I'm just a high school chem student, but I've got this question and I couldn't find the answer anywhere online.
I was reading an article about some futuristic "artificial gill", which kinda sounds like BS to me, but it got me thinking about this. 

http://www.smithsonianmag.com/innovation/student-claims-have-designed-working-artificial-gills-180949465/

We were just learning in class about electrochemistry, and we just did electrolysis, so I was just thinking, instead of filtering the water, what if they used electrolysis to produce O2 gas?
There would still be hurdles, like diluting the 100% O2 to about 20% gas by volume, storing the necessary electric power to drive the reaction, and making sure that the unit wouldn't blow up, but could it theoretically work if we could overcome these other issues? 

Basically, could you produce enough O2 gas to survive, through the electrolysis of H2O, in a unit small enough for a man to use?
In scuba diving the main issue is pressure. To be able to breathe, you need to counteract the pressure of the column of water on your rib cage. This is why divers breathe from compressed gas tanks. Otherwise their rib cage and lungs would be crushed by the pressure. At higher pressures, air becomes too viscous to be breathed without too much effort, and the build-up of dissolved nitrogen in the tissues leads to narcosis ("being drunk"), so the oxygen has to be mixed with helium, that is much less viscous. So, with theses 'artificial gills' or with electrolysis, you still need a tank of helium, and a powerful compressor.

Overall, it is much more convenient to carry tanks of compressed mixtures of gases. Compressing gases is a form of energy storage (that could even power cars). But if you are in a nuclear-powered submarine, then you would be better off doing electrolysis.