Disclaimer: I am NOT a doctor and what I say should not be taken as medical advice.
However, it is our (my DH and I) understanding that when you get dehydrated in the sun, you are losing electrolytes, the major one of which is sodium. The sodium balance in the body is pretty specific and its declining levels with dehydration due to heavy sweating may be a problem. This is one reason why electrolyte replacement drinks are so popular. Unfortunately, they cram a lot of sugar in them, too.
On someone's medical website, years ago, I saw a recipes for a Gatorade substitute using diluted orange juice. It also called for a little salt. I can't find it now but saw this recipe
while I was looking for it today.
We originally researched this back when my hubby was doing LONG bike rides (over 50 miles and eventually 150 mile race) and then ultramarathon running (over 30 miles). Pure water does not cut it for rehydration and keeping the body going in such extreme exercise. The body needs some sugars and salt in an easily digestible form (because it is still working), hence the sport drinks and sport gels for athletes.
Now, for gardening, obviously we are not working that hard. However, it is hard work and I sweat a lot, so I do not make an extra effort to restrict my salt. I naturally do not salt my food heavily but I don't buy sodium-free canned tomatoes or beans, and I do use table salt on my food.
I do not have really high blood pressure to worry about, though. If that is a worry for you, then changing your salt intake based on what I say (as a non-doctor) may be ill-advised. Especially since my blood pressure was slightly higher than it should have been at my last doctor's appointment AND I got overheated myself in early April. So what do I know?!