I used to drink a similar concoction when I was on a low carbohydrate diet, but without the sugar. I would also add a little bit of salt substitute called "No Salt" which is potassium chloride. Or as a tastier alternative, sometimes I would just dissolve a couple bouillon cubes in some hot water and drink that. The reason I did that is that a low carb diet can cause your electrolytes to be depleted.
The "rehydration salts" are designed to replenish electrolytes (sodium, potassium, etc.) that are lost due to exercise, or to losing fluids due to vomiting or diarrhea, or to diuretic drugs (such as kava). However, the salts themselves don't actually re-hydrate you: they just replace missing electrolytes. You still need to drink lots of water, because obviously only water can replace the fluids that are lost. If you are dehydrated, and you just drink 1 cup of "rehydrating salts" (regardless of whether it is a US or UK cup lol), you will not be rehydrated.
So, I think drinking this stuff might help alleviate or prevent some symptoms of dehydration, but it is not a substitute for water, and you should use it in moderation. It probably won't completely stop the diuretic effect of kava. In other words, you will still pee, but the antidiuretic effect of sodium might help you not pee as much.
Another thing to keep in mind is that if your body is too high in electrolytes, it can be bad for you also (it can even be very dangerous). Please remember the lesson from this fictional dystopian hellscape:
