I would not dream of asking for water at my mum's and it's not something she would offer. I certainly wouldn't get up and help myself
Wow, just wow @DDIJ So you would make yourself sit there getting thirstier and thirstier and making a martyr of yourself, for something that isn’t rude in the first place? Is this the home you grew up in? Did you not help yourself to a glass of water or other drink when you lived there? Why on earth do you feel you can’t ask your own mother for a glass of water? Is she really so scary? In what family dynamic/culture would this be considered rude?
Sorry but I DO think it is rude to jump up from the table, burst into someone's kitchen and start rummaging in cupboards looking for a glass and help yourself to a drink
You ask if they mind you going to get a glass of water. Are you afraid of your parents?
so, it would be rude to start demanding water
It is rude to demand. It really isn’t rude to ask. It sounds like you need to do some assertiveness training.
I am utterly gobsmacked at the meekness and overthinking sensitivity of some posters who allow people to walk all over them, especially for something as simple as asking for a glass of water 
This is the 21st century where women have a voice. We aren't meek downtrodden creatures that the older generation can control.