1) cell construction- very nice to hear that a wet cell of these dimensions was created, it has the square inches to handle the amperes.
a)what is the electrode configuration? i am assuming it is in a +-+-(...and so on) configuration.
b)your large surface area means you will need very little KOH to pull many amperes. be careful of this condition- if i were you, i would install something called a "marine battery isolator". it is a simple device that runs two batteries in parallel- and the alternator only ever charges one at a time. obviously, the 'lyser would be wired to the secondary battery. pulling a constant, extra 30 amperes may burn out your alternator rather quickly.
2) water concerns- at the BARE minimum use filtered tap water. best is high quality distilled but i'll admit i even buy the CVS(american corner drugstore) distilled.
a)your high water content will help with dealing with any excess heat that may accumulate.
3)flow meter- that is a fairly reliable, fairly accurate (a good way to say is "its accurate enough") method for measuring flow within the system.
4)conditioning the system: be sure to condition the system for a week or more. really give her a go- and be sure to record output data in a fairly methodical manner.
5) bubblers and flashback arrestors. i reccommend a total of three bubblers but thats simply my technique. two at the 'lyser and one right before the intake. you REALLY dont want vaporized KOH or NaOH coming into contact with your aluminum parts.
6)get a project folder- the more raw data, the better, cant stress that enough. pictures are worth a thousand words, and extremely helpful in figgerin stuff out if/when u have a problem.
other than that, welcome and happy tinkering.