Ideally, you could quickly get them to a good duck vet - just want to write that on the thread for anyone reading this in the future.
Could be some kind of toxicity, yes - I would change the food - you are trying some good things, like the electrolytes.
I would bring them into a pen with lots of fresh water available to them.
The problem could be in their environment. What's the weather been like there? Sometimes during hot, dry spells botulism is more common. One thing that also came to mind was some other kind of toxin - perhaps from an algal bloom in the lake.
Or could someone have possibly dumped something toxic in the lake? Are there boaters? I don't recall how large the water body is, and I admit, I am a little tired right now.