Short answer: Whichever fits the character
It could do either way, so it depends on the way the character view the magic as working.
Choosing to go with conjured water has the benefit of always being available, no matter how dry the environment is. Using the water present leaves yourself open for a lot more compels, as well as opens a lot more declarations, useable to influence the magic.
* "I bring forth a mist to hide my escape, lucky me that there's a
Fountain full of Water just to the side!" [rolls alertness for the declaration to tag in spellcasting]
* "Boy would that mist you usually call forth have been really handy now, if it weren't for the
two-week dry spell that's hit the place!" [slides player a fate point]