@MrsFionaCharming
If you are feeling dehydrated despite drinking the advised large amounts, you are lacking sodium potassium and magnesium - the latter being something a lot of people lack.
LCHF flushes these minerals out of the body as the retained water from high carb consumption and BIWI has explained we need to up our intake of these minerals to compensate. Salt has had a bad rap but without salt, we would quite literally desiccate and die. But it is a balance. I, for example, need and consume a lot of salt. For others my consumption would be ill advised.
In order to absorb the water you are drinking, you need to raise your body’s osmotic potential otherwise some of what you’re drinking is not being absorbed and is being expelled directly as pee. Think of it as watering a plant, which has its roots wrapped up in a plastic bag.
For hydration, I drink 3 tsps sole water diluted in a big glass of water. I would try less at first as if you drink it in a form, which is too concentrated, you will get the runs. To this, I add a glum (1-2 tsps?) liquid ionic potassium. I bought this off amazon the first time but found it cheaper from metabolics.com. This is a top class supplier of vitamins and minerals btw. If I need magnesium this is difficult for the body to assimilate, therefore, I use a transdermal spray magnesium. This is the sole recipe. wellnessmama.com/12158/make-sole/. These work best for me. But there are also other oral potassium / magnesium supplements.
The other thing you can do if you don’t fancy sole is to drink bovril or marmite. Both are great for hydration. The body needs to take these on in liquid form to increase the osmotic potential.
As for preprepared electrolytes, you’re right. They have a lot of sugar in them. These are used for rehydration for malnutrition and acute dehydration.