They want to split the water into hydrogen and oxygen by electrolysis. Not knowing the exact process, I'm not too thrilled about the idea except as a short term solution. I think Mars is a place where it would be better to collect water, instead of burning it up as a raw fuel. If it was a water planet that would be a no brainer. Mars could turn out to be a water planet with all the water below the surface. But if it isn't there, water is one of the most expensive things to ship around the solar system, because of weight and density. It might end being needed as a way to keep things clean, a very unexotic use for water.
The land on Mars is a toxic chemical dump, starting with perchlorate compounds, and plenty of other energy laden materials that can be used to provide vast amounts of chemical energy, including fuel for fuel cells, which could be built as big as football fields. It would be better to harvest water from all the industrial processes that need to be done to survive on Mars. There might even be rocks with oxygen or water as a chemical component that could be tapped. Energy intensive, but energy derived from chemical reactions is no problem on Mars.
Fuel cells are very interesting devices as electro chemical engines. They can be run in forward mode to produce electricity, and in reverse mode, taking electricity in and producing valuable resources, including water. Mars is venting water vapor into the atmosphere, which probably would be a better source, though harder to harvest, as some of it probably escapes the planet entirely.