Just water?
From your last post, you said you drank 200oz (6L) over 5hrs. I’ve seen sweat tests for running in the heat range from 0.5-2.5L/hr. The smaller values were small women, the larger ones were larger men. So it’s possible that you aren’t drinking enough.
I’ve also heard that it’s hard for the body to ingest >1.5L/hr (when running). Maybe it’s diff for yardwork, maybe not. If not, this suggests there’s a limit on the hours those big sweaters can work before dehydration starts. Prehydration and slowly down are the only things that can help.
And lastly, sodium. Sodium sweat rates seem to range from 200-3000mg sodium per L of sweat. Multiply that by the sweat rates and you can see how varied people are. People survive half marathons without caring about sodium intake because the run is short and the American diet has plenty of sodium. But 5hrs of work is no longer short. And doing it in the heat makes it far worse. You must have sodium in your fluids, and it may need to be A LOT. Adding some sugar will also aid absorption.
I just bought these at-home sweat test patches to see how they work. People are too varied to just assume you are in the middle of all these ranges.
Gatorade, Gx Sweat Patch, Pack of 2
https://www.amazon.com/dp/B0BNLZSRNN
We tested this stick-on wearable that measures your sweat rate and sodium levels to see if it can help you optimize performance.
www.runnersworld.com