Yeah, if you know you drink a lot of water, and experience shows you need to supplement what the average host gives you with your own supply, one option is to mention it to the host beforehand, and see if they can give you a bigger glass or put a refill pitcher right on the table or whatever. But, of course that really depends on each situation. Like if you don't know your host well, it might be kind of weird, or an awkward host might then draw more attention to it--"I put some extra water on the table for Alice, but everyone can help themselves!" And you don't want to make it seem like it's some kind of dire necessity, like you're going to collapse if you don't get constant water or something else the host needs to watch out for.
So I can see how people just have to navigate each situation as it comes, but I would say as a general rule, aim for being discreet while still meeting your needs--like if the host has obviously taken the time to put together a nice table, avoid just plopping your lime-green plastic water bottle on it for the whole meal, you know? It's not a deadly sin by any means, but I think it's part of being aware of the situation around you, and treating the outing as a special occasion. Sometimes people can get really desensitized to stuff they do every day, and they don't think about how incongruous it is to other people around them--like I remember so many times new parents checking their baby's diaper at the dinner table and discussing it in detail, like they do all the time at home, and I at least am getting really grossed out because I don't think about that all the time. And I don't think it's really appropriate to subject other people to that at a meal time, but the new parents are just sort of used to it.