Just water. As said, less cleaning issues. Though don’t let it sit around stagnant at home in the warm to grow bacteria. Chuck in fridge or empty and put in freezer.
I used to fill with high juice and salt, but ran out of juice to fill it once, just did water and being doing that since, not noticed a difference, or if anything seem to feel better.
I don’t buy into tablets and gels. A lot of artificial stuff, and sugar. Stuff especially designed to “hydrate”, which is err, what water does.
Sugar… I’ve mentioned before somewhere about a medical guy interviewed on the radio (runner and maybe cyclist, can’t remember), and he swears he got type 2 diabetes from all the gels and energy drinks/bars he was consuming.