User blog comment:IamJakuhoRaikoben/Chapter 271: Lucy vs. Flare, Review./@comment-108.38.11.56-20120218151209/@comment-3290324-20120218162350

Actually, she was summoned when Lucy was talking to the spirit king w/o water, so she is summonable w/o water, but as I said earlier, whether she chooses to fight when she arrives is a different story. In addition, she can create water by herself so while obviously a large body of water would help, it's not needed. Don't forget that she was not summoned in a large body of water at all during the S-Class Trial.