Not something you need to worry about in the Lakes, surely?
Well yes it is...
You need to drink plenty, especially on warm/hot days. Dehydration, even by a small amount, causes brain impairment, and may be a factor in slips and trips in the afternoon, the biggest cause of callouts for us. In more extreme cases it is life threatening. It can be linked with hyperthermia and uncontrollable rise in body temperature.
The best drinking water is tap water taken with you. Easily 2 litres or more might be required. If you run out, then it should be safe to top up from moving, well airated water. Check there's not too much by way of livestock about, and if it's rained recently, then be extra careful.. faecal matter can be washed off the surrounding grass and transported in the water. The higher you go, the lesser the risk... Cryptosporidium is probably the main concern.