I don't know about the huge energy requirements to split H20, we used to do it on a regular basis on the subs I've served on. The 'box' is about the size of a refrigerator and separates to it's two components, Hydrogen and Oxygen, both stored at 5000psi in tanks, we considered the hydrogen to be a dangerous waste byproduct and would pump it overboard back aft so it mixed with the turbulence from the screw preventing 'sniffer' tracking and a big hydrogen wake that could be detected from satellites. Unfortunately I don't remember the input requirements from the power panel that fed it. We used it to replenish the oxygen in the internal atmosphere. I know we had to dump that storage tank after every 8 hours of running the cracker and it took about 4 hours to vent it.
I do know that Hondas wall mount setup ran off 110V but they never gave a CFM rating on the dual scuba tank sized generator. Every submarine in the US Navy and every other navy in the world has them. We would process water from the 500 gallon per day distiller, the 5000 gallon a day distiller was used for potable water. I doubt if I have my SIB's [Ships Information Book] that showed all those systems including power and all the qualification information anymore. Generally we would run one of the two generators once a week to keep the O2 levels at 11% vs the 19% we breathe in the rest of the world.
The last sub I served on, Will Rogers SSBN659, went to the breakers in the late 80's along with almost all the original SSBN's from the cold war. The only ones left were the ones modified for seal team and special warfare use.
Way back then I remember reading intelligence briefings about other navys developments and the some of the other countries development of power systems for their littoral submarines [shallow water and coastal] was very impressive. I know two countries that have been using hydrogen fuel cell technology to power them for over 20 years and it's not the US or Soviets, those boats used to throw the fear of god into the sonar operators, they were undetectable except with an active ping.
Hydrogen is the future, we just need to find a cheaper way to get it out of the water.