if it would be possible for the water logia to exist, then it would be able to beat all df users. This is because ALL types of water, affects df users(stated by oda in a sbs) but it may actually be the weakest as well. Because it is also possible for that person to kill themselves useing the fruit because they might still be weak against water, and if they are made of water....well yeah..
But if a water logia is not weak against its water, and its water affects other df users, then it would be crazy op.(as they may be able to manipulate the body as well, as it is made of water.