Mom has become dehydrated several times, last time she had hallucinations. How can I get her to drink?

Summers in Florida can kill. Keep her in air conditioned space if you can.
Use electrolyte drinks like Pedialyte or Gatorade. Good Luck.

Feed her foods that are high in fluids - juicy fruits (watermelon, cantaloupe, grapes), cool soups in summer and hearty soups in winter. If she's not diabetic, try milk shakes - the sweetness usually is appealing. Fruit salads are good too.

Avoid anything high in sodium, not only because it's not healthy but because it will accelerate absorption of fluids instead of allowing them to remain available for hydration.

Another thought although I wouldn't rate this as high as something more healthy such as fruits and soups - homemade popsicles with fruit juice, sherbet or even ice cream. Again, the drawback with these is the high sugar content and sometimes sodium content, so it would be a balance between how much healthy food she eats and how much not so healthy but more fluid rich food is offered.

Some herbal teas are a possibility as well. There are also fruit juices that are better than the typical canned or frozen juices. They're usually more expensive but are generally more pure than ones which are pumped up with sugar. Cider is another possibility.

There are flavorings available for water. Do you think that might help? The beand name I think is MIO.

My husband used the water flavors like glad mentioned and now he drinks more water than he use to. There are several brands even the strore brands are good. You just put a couple drops in a bottle of water and shake it. They add just a hint of flavor.

High-fluid foods. Tomatoes and lettuce on sandwiches. Soups. Cereals with milk. Watermelon. Other melons. Peaches. Pears. (Fresh, canned, or frozen.)

Beverages she likes: juice, coffee, tea, herbal tea, lemonade, gatorade, cranberry or other juice with tonic water or soda. Beer. Root beer floats.

Popsicles (purchased or homemade). Pudding. Jell-O. Strawberries frozen in juice thawed over ice cream.

She could stay hydrated without actually drinking plain water.