How do I get my mom with dementia and Parkinson's to drink? She's dehydrated and refuses to drink anything.

This may sound like it wouldn’t work but I started my DH aunt on the small water bottles years ago. They are small so they are easy for her to hold and are less intimidating because they look like not that much water. I would have one with her. Never cold, just room temp. She would drink it right down.
Another thing to try is a straw, as in a milk shake or protein drink with ice cream and whipped cream on top.
Aunt would no longer eat soup. I think it had something to do with spooning the liquid to her mouth. I couldn’t get her to drink soup either although she loved hot chocolate and coffee. Watermelon she really likes and also likes to peel the little cutie tangelos and suck the juice. Again a small item and easy to peel was a plus.
Jello cubes with whipped cream was always a hit.

Dementia is so strange - my mom has it too. I marvel at the things they remember and/or come up with. Why does she have to remember the correlation between drinking and peeing yet can't remember the dehydration part of the equation?? It's not difficult to reason with her at this stage - it is impossible.

I like the idea shared about foods with high water content. I have trouble with my mom not drinking enough too. I try to do small things to sneak in more liquid without her noticing, in addition to harping on her to have some more of her water. They just don't care or understand the impact on their health.

Good luck. Do your best but try not to fret too much about it.

I am so sorry you and your Mum are facing this challenge.

Will she eat soup, melon, popsicles, watery foods?

Had she had IV rehydration?