Max, you are spot on with your product acessment, it's the water. And as you stated, the B is for color and the Creatine to assist in passing the urine validity test, though for the life of me I have yet to figure out why they add the Creatine. See, it takes Creatine 24-48 hours to metabolise into Creatinine, the metabolite measured to acess whether or not the sample is dilute.
Since you've already passed 2 home test, I also agree you have little to worry about. Good luck and post your experiance and results. N2