I suspect my issue is dehydration plus hyponatremia as a result of heat stroke. I know that I get dehydrated, because that's what the docs at the ER always say. And the nurses, as they struggle to find a vein for the IV.

But I also get the nausea and jitteriness associated with hyponatremia. I suspect that I sweat so much that I cannot realistically replenish it by drinking water, but that the water also dilutes my electrolytes significantly.

I was going to say that I can't drink Gatorade because of the corn syrup, at least not frequently, but then I remembered that they used to sell it as a powder, which couldn't possibly be sweetened with corn syrup. Turns out they still do, and they also sell a "natural" version that doesn't have corn syrup. (Probably; they don't have the ingredients list online.) So that's something to consider.

Swimming as a way to control heat is a good point, too. That's something I hadn't considered.