But it’s not when we’re talking about this in the context of weight loss/gain. When people colloquially say “gain weight” or “lose weight” they almost always mean gain/lose fat and/or muscle. Water weight is neither of those, and is generally bounded (unlike fat loss/gain).

Whether you gain or lose weight through water weight has little to do with calories at all. If you take creatine, for example, it’s pretty normal to gain a few pounds of water weight until your creatine levels equalize in your body. That doesn’t mean that your body’s “metabolism” changes or something based on the calories you ingest. You’ve just got another pathway to gain/lose apparent weight, albeit temporarily.

I'm going to guess people find you overly pedantic?