Is the nutritional drink Boost considered a good way to add fluids to the body when the senior won't drink much water?

I'm sure it is better than nothing. Would she drink Gatorade? I personally don't like drinking water, I prefer sparkling mineral water with a tiny bit of cranapple juice in it or diluted Gatorade. Maybe give that a try.

Any of these might also work
Popsicle
jello/gelatin
juice, I would dilute it a bit but you still have the flavor
fruits
Any fluid is better than none.

Yes, any liquid is good.

Pedialite Is also good. They have Popsicle.