Any idea at what point the creatine and gravity levels start getting compromised from drinking too many fluids?
No idea. That would depend on what your "baseline" S/G and creatinine levels were prior to diluting - baselines vary from person to person - and youre not going to know your baseline without testing your creatinine and S/G with a validity test strip. These test kits are handy, inexpensive and are found online by doing a Google search for "drug adulteration test kits".