It depends where you are, I would say the heelands are as good a place as any but you can't really account for what's happening upstream, a dead Stag decomposing in a stream as often happens when they get down to drink and fall over in a confined space, they can't get back up,
Wales, unless you know the farmer has NOT dipped his sheep recently then again a bit icky as you can't say for sure what's in there.
At the least carry some puritabs if your going to be adventurous.
In a situation where some one is suffering from heat you could sit them in the water or stream lake or sea, that'll get the temp down, but always try and carry sufficient water. The last thing you want to be doing is going Man down because you did a bear Grills ?
While I'm here I'd might as well point out that there's tap water on the red at Dalby, I pointed 3 teenagers there in the week as they were banging out the red with jump bikes ! No seats no T shirts on ? And them stoopid baseball caps that make the kids look like Ruprect !
On and carrying no water between 3 on the hottest day of the year ?
It's in the field on the left where campers sometimes tent out near the blue thunder box in the field, you can see it as you pass by if any one is interested ?