Drinking water is the cheapest and easiest way to maintain your vocal health. If your voice is not sounding top notch the first thing you should do is take notice when you last had a drink of water, two is noticing your vocal behaviour and three is to take a look at your vocal environment.
The research says dehydration will affect the sound of your voice and surprise, it says improving your hydration will improve your voice.
There are two ways to keep your body and your voice hydrated. First is ingesting water e.g. a glass of water. This keeps your body functioning at optimum levels. The second way is to maintain the water levels on the surface of your vocal folds. This will keep it well lubricated allow smooth vibrations of your folds. You can do this through humidifiers and nebulisers and probably keeping away from drying environments.
This paper looked at a number of research papers which highlighted these important hydration factors. Caffeine does not look like it affects your voice and breathing through your mouth will dehydrate your voice.
Article Review: The Effect of Hydration on Voice Quality in Adults: A systematic Review. Maxine Alves, Esedra Kruger, Bhavani Pillay, Kristiane van Lierde, and Jeannie van der Linde. Pretoria, South Africa and Ghent, Belgium. Journal of Voice 2020.