It doesn't matter how much water you drink if it doesn't get into your cells. Potassium is the most important electrolyte for this purpose; you can add KCl to your food instead of / in addition to salt. (Or just eat more potatoes with skin) Use cronometer to estimate your average daily intake from food, and supplement to reach 3500-4000 mg / day. It's not as effective to take potassium on an empty stomach; you will absorb it faster and pee it out. Betaine is a cheap and safe supplement that acts as an osmotic buffer, which also helps protect cells against dehydration by drawing in fluid. It is safe to take a few grams of betaine a day.
It is also possible that you might need a bit more sodium, but that is less common. Magnesium deficiency makes your body waste potassium, so if you don't get enough from diet (which is common), supplement up to 400-500 mg/day, preferably in any other form than MgO. Lastly, pumping potassium into the cell requires ATP, so if you have a chronic disease associated with poor mitochondrial function, it may be necessary to address that (which is a long and complicated process, so try the easy stuff first).
Hope this helps.