I struggle to get my father to drink enough. I give him drinks he likes but short of nagging I can't get him to drink much. He usually drinks 3or 4 glasses a day instead of the recommended 8. He is frail and elderly (90yrs old). He drinks fruit juice or coffee. He sleeps in a chair most of the day and sleeps in a bed at night.
Maybe I'm being very ignorant, but three or four glasses of water a day doesn't sound too bad! I think there's a large number of people who aren't anywhere near his age who don't drink that much!
As for the fruit juice (again, great that he drinks it at all!), I wonder if you could dilute it by half, so that he gets more water and less (fruit) sugar in it?
I fear it is very common of those of his generation not to drink much water. Firstly they may have grown up at a time when tap water was not particularly safe to drink, or it simply wasn't the custom to drink it. My 93 MIL NEVER drank water 'willingly' - or, indeed, at all! She didn't drink fruit juice either, just tea, wine and G and T!!!! She's remarkably healthy in her body for all that!