To be beneficial, fluids injected into the human body, among other considerations, should be isotonic. “Isotonic” means that the fluid has the same concentration of salts as the body’s blood and cells. Fluids with a greater quantity of salt are called hypertonic, and those with less, hypotonic. Sterile water has the lowest possible content of salt (none) and occupies the low end of the spectrum for hypotonic fluids.
There is no medical reason to inject water into the buttocks. Hypotonic injections are extremely painful. The water would cause immediate hemolysis (bursting) of the blood in the tissues, and lysis (bursting of the cells) of the solid tissues as well. This is because of the marked difference of salt content. If more than 10 or 20 ml were injected the patient would end up with a seroma, which is a localized accumulation of degenerating blood, tissue, and bodily fluids. Although the seroma would eventually be reabsorbed and ultimately heal, there would be a danger of infection and abscess formation.
It occurs to me that you might be considering injecting water into the buttocks for cosmetic purposes (a homemade “butt” lift). This would be painful, extremely dangerous, and would have no lasting effect on the contour of the buttocks. Actually, that’s not entirely true. If the area becomes infected, there may be scarring, retraction and permanent unsightly deformity of one or both buttocks.
The reference defines hypertonic and hypotonic fluids.