User blog comment:B214/Chapter 499: Gray & Juvia/@comment-7418318-20160829143550/@comment-25111584-20160829145434

There are somany BS things in the Chapter and a promise of so many more to come... and yet, surprisingly, ignoring Water Body is perfectly logical.

Ice Lock does not allow its victims to escape. Water Body would allow Juvia to escape, hence its use is prohibited. Honestly, it doesn't even have to be explained. And, yes, Magic is not sentient, but it is Magic, so I don't think there's much of a problem with Ice Lock restricting the use of Water Body.

Of course, there's also the fact that Water Body is one of the worst defenses against an Ice Mage as it just makes it easier to instafreeze Juvia, although I doubt Mashima thinks that far...