It's the not the cost of obtaining the water that makes water less viable, it's the cost of storing superheated water. Water has this pesky ability to expand 1600 times larger when it goes over 212 degrees Fahrenheit. That means that if not handled carefully you get deadly steam explosions.

Hot sand / crushed rock doesn't have the same problem. If you read in the linked article it says

> with the sand heated to somewhere around 500-600 degrees Celsius (932-1112 °F).

That would be extraordinarily hard to do with water as you'd need significant containment and safety measures.

If that water has to come from underground, yes, it would be cheaper and easier to just grab surface stone and crush it for material to make a heat battery.

Also, water tends to make for a horrible heat battery as it is much more thermal-emissivity than rock particles. You need all sorts of additional insulation to retain the heat, whereas the sand will insulate itself.