I do drink plenty of water, maybe too much even, am in good shape, and have no medical conditions of any kind. Weed greatly exacerbates this issue for me, probably from the fact that it reduces blood pressure, but this can still happen when I'm not smoking for extended periods.
Recently, I figured out a way to short circuit the onset of a dizzy spell. Hyperventilating completely nips this in the bud, likely because it increases blood pressure [1]. I'd recommend trying this technique to anyone that's otherwise healthy (I am not a doctor) and coping with this issue.
[1] https://www.ahajournals.org/doi/full/10.1161/01.HYP.00000523...