Kallog
There's a difference in ambient pressure between the top and bottom. That's needed to make bouyancy work. So you're using more electric energy creating HHO at the bottom than you would at the top.
the inside of the containers are not subject to the water pressures outside of the containers.
the container itself will not implode due to pressures outside of the container if it is built strong enough to withstand the highest pressures that it would be subjected to.
the water inside the containers will only be subjected to atmospheric pressures or the vacume that is supplied by the carburetion of the engine used to burn the hydrogen.
if you tie the end of a water hose to a plastic bag , then insert two electric wires
(that have the ends stripped of insulation)
through the water hose , then fill the plastic bag full of water , and sink this in a swimmng pool , then yes the underwater pressures would require more electricty to convert water into HHO.
but if you use a metal container or a continer that would not bend under pressure under water then the same amount of electricity would be required as if the conversion from water to HHO were being done in an open container above the water.
Im sure you know this as you say you have some engineering experience.
think of a submarine they go thousands of feet under water
but the people inside the submarine are living in atmospheric pressures inside the submarine.
for an example the outside skin of a submarine at a depth of 3000 ft will be apx 1300 psi while a sailor inside the submarine can drink a glass of water at 14.7 psi or 1 atm.
After it's patented the cat's out of the bag.
with something like this the cat might live longer if you let it out of the bag before you attempt a patent , otherwise the bag and the cat might end up in a storage area somewhere along with all the other cats in the bag.
here is an example of someone who uploaded a video to youtube about usng the buyoancy of hho to gain the lost energy back.
http://www.youtube.com/watch?v=J2ZDmotdwpc 
