Go to Youtube.com and put in GE90 test and watch the video there. Water at the rate of 4.5 tons per MINUTE just produced a lot of steam out the back!
Forget about being impressed by the apparant steam out the back. It ain't what goes through the fan and out the bypass that's the worry. As stated above it's the amount of water that has to be converted to a gas before it hits the combustor that's the worry. This part is unseen in the videos and can only be assessed via the performance chacteristics of the engine right up until it stall/surges. Some of the engine tests even go so far as to only spray the water into the core and leave out the eye candy effects out of the fan duct.