Dehydration; how do I get the assisted living facility to realize they need to make her drink water?

They can’t make her drink water. I would find out what other beverages she likes, get her popsicles and frozen fruit or ice pops that constitute water and help toward achieving hydration (non diabetics). Diabetics can shop for Crystal Light” no sugar flavors too to help water be palpable. Google “Foods that metabolize  to water”& review to see if she likes any and can properly swallow. 
It’s hard for us to remember to hydrate ourselves! But suffering dehydration and being elderly is preventable, thus good luck in hoping her hydration improves! 

Mother won't drink plain water, but will drink a half-Coke half Diet Coke with lots of ice. She's diabetic and I know this isn't the best option for her, but she's 88 and can manage her diabetes with the sugar.

I know in elderly women, esp., that curtailing drinking also curtails runs to the bathroom. Mother is chronically dehydrated, I think most people are!

My mother won't drink water, but she loves ginger ale. Ask if the AL has a refreshment cart visiting residents

My mom won’t drink water either, can’t make her drink water. Even when she lived with me,,I would fill up her water bottle before I left for work, and it would be filled the same when I came home...

The AL can't "make" her do anything. Unless your mom has been declared incompetent, she is free to refuse something she doesn't want.

What DOES mom like to drink? Can you make sure she has access to that?