For rucking in North Carolina summer heat I used the Oral Rehydration Salts (ORS) packets we could get from the medics. I mixed them with about twice as much water as the packet says to use.

https://www.amazon.com/Jianas-Broth...1-1-12d4272d-8adb-4121-8624-135149aa9081

Not sure if you can buy them in bulk like we used to get them in the Army.