I have only seen the movie but I am wondering why Mark Watney continuously had a flame to produce water. Once he had enough water for the crops he would only lose a tiny amount of water vapour in the air whenever he went outside but wouldn't it have been better to conserve the oxygen to breathe instead of using it to produce even more water? In other words, wouldn't he have ended up making far too much water?
In the source novel, he initially stops making water around Sol 60, having determined that he's met his water needs for the foreseeable future.
I finished making water some time ago. I’m no longer in danger of blowing myself up. The potatoes are growing nicely. Nothing has conspired to kill me in weeks. And seventies TV keeps me disturbingly more entertained than it should. Things are stable here on Mars. It’s time to start thinking long-term.
Around Sol 112, and basically at the insistence of NASA we see that he's using repaired the water reclaimer, preparing for his long journey and as backup in case his water supply becomes depleted.
Okay, technically I’m lying. The plants aren’t entirely water-neutral. They strip the hydrogen from some of it (releasing the oxygen) and use it to make the complex hydrocarbons that are the plant itself. But it’s a very small loss and I made like 600 liters of water from MDV fuel. I could take baths and still have plenty left over.
NASA, however, is absolutely shitting itself. They see the water reclaimer as a critical survival element. There’s no backup, and they think I’ll die instantly without it. To them, equipment failure is terrifying. To me, it’s “Tuesday.”
But yeah, Mark's thinking is much like yours; "Hey NASA, what the hell am I going to do with all this water?"