This sounds like a really bad idea IMO. Define
gulp? At that how long do you intend on holding onto this bottle of water? The inner junkie in me is screaming obtain bacteriostatic water. That would be your best bet on storing MDMA in liquid form.
Also... the process that would ensue to just do this,
safely, would be a tremendous effort. I'd imagine you need to do some math, obtain syringes, get a good scale, micron filters, and as mentioned bacteriostatic water. You'd need to know weight distribution of the MDMA HCL vs binders as well, which would probably vary pill to pill.
I mean really, to me, that is way too much work when you could just break that pill in half and enjoy yourself. Save the water for when ya thirsty.
Edit : All in all this is not a good idea. If you intend to dose your friends out or whatever it just seems bad all around. It is such a subjective question to ask what a good ratio of MDMA vs X fluid OZ would be? Everybody is gonna be different.
Edit 2 : It would be easier done with a crystalline or powdered MDMA sample. Then you could really get down to the nitty gritty. Like one gram per 16.9 Fluid Oz.
http://www.bluelight.org/vb/threads/500605-Solubility-of-MDMA < Older thread in which in slightly discusses this proposed idea. It also has a post from Shambles which is lol worthy.