Why would you want to do that? It is a medical test not a morality exam.
It makes you pee a lot, thereby 'flushing' your system.
Well, I haven't heard in my 12 years of medical career that water flushes out alcohol from body.
People advise to take plenty of water after alcohol consumption because of the following reason.
Alcohol acts like a diuretic (a diuretic means substance which makes water from body to be excreted more in urine).
So, when one consumes plenty of alcohol, he is in danger of losing much amount of water from body and undesired consequences.
Hence, it's advisable to drink water to avoid dehydration.
Water has nothing to do with 'Flushing' the alcohol out of body. It's a pure myth.
I think, one should be having enough knowledge about what he is suggesting others, especially when a serious matter such as immigration is concerned.