User blog comment:FairyRaven/Fairy Tail Chapter 428: When We Take Different Paths, Review and Commentary/@comment-76.121.234.236-20150415065247/@comment-7418318-20150417062622

But she is no slayer. Sure she can turn her body into water, can use the water in her surroundings and maybe even absorb water (though I don't remember seeing that so far), but for that she has to make a conscious effort.

If she just sits there without activating her magic, she has a completely normal body.

I also think that she might not be able to consciously change the temperature of her water. The only instance when Juvia attacked with almost boiling water was when she was in the grip of jealously. But even if she can do that consciously, it's still require her to put in the effort, which she clearly wasn't doing.