NRG Fitness Water is a drink from a garden hose that only appears in Diary of a Wimpy Kid: Old School.
Greg and Rowley sold lemonade but after many failed attempts (the lemonade being too sour, too sweet, and too watery, the same cup being used for each customer, the water used for washing out the cup came from the hose) they decided to instead just sell water to save money and make it easy for them.
They put water from a garden hose into an inflatable pool and advertised their (totally normal) water as NRG Fitness Water to make a profit. They advertised that it was a fitness drink when in actual fact it was just normal water.
They told a biker they'd pay him to say he got fit drinking their water which didn't work out because he was not interested. A slightly tubby, or "overweight" as Greg put it, guy heard he could get paid and asked to be paid to say he drank it. They told the man they'd pay him to say he didn't drink it so that no one thought they'd get fat drinking it, which didn't work out either.
Greg got Rowley to exercise in front of their booth to make it look like Rowley got fit from drinking it but Rowley wasn't very good at exercising and didn't change shape from it, so that idea also failed.
Then they decided to bring there NRG Fitness Water to the town cleanup (that happened as a part of the electronics-free weekend that was a petition started by Susan Heffley and passed by town hall, which each town citizen was told to come too) and sell it there, and they discovered the water fountain was broken so people would need to buy from them if they wanted water. Susan Heffley lectured Greg that they shouldn't sell their water because they were working for free, and, during her lecture, some volunteers poured the water on plants. At that point, Rowley was helping in the cleanup and Greg ended up helping in the cleanup too. That was the end of selling NRG Fitness Water.