Water weight: Drink 1/2 a gallon of water

What's your actual question? Yes, if you drink or eat anything and weigh yourself straight after you will weigh more on the scale.

Sure it makes sense. If you were to put a bucket on the scale, weight it, then pour a half gallon of water into it, you'd expect it to immediately weigh about four pounds more from the weight of the water.

I am lost as to point of this - of course if you weigh yourself, then drink half a gallon of water then re weigh you will weigh about half gallon more.

But obviously that isnt your body weighing more and obviously that water wont stay in your body unless you are in renal failure.

and obviously your weight fluctuates over the day as food/fluid goes in and out.

That's why people suggest weighing with as many variables as possible evened out ie same time of day (whether daily or weekly or whatever) go to toilet first etc.
and still expect slight fluctuations

what was the question again?

I think..... what you mean is that you imagine drinking water will make the scale go down eventually, and that is sort of true. Drinking lots of water after a hard workout or consuming a lot of salt or even being bloated from your period can help your body "let go" of the excess it is retaining, but it's not much in at least two of those 3 cases, and it's still EVENTUALLY. It's not gonna happen in a couple hours, think more like several days of getting plenty of fluids can* have some* effect.

Water weighs the same whether it is in a bottle or in your stomach (until you start to process it, of course). So yeah... any time you eat or drink anything, the weight of that thing gets added to your weight. At that point it isn't even "water weight" (which generally relates to someone retaining fluid), it's just... physics.