This is from a chemistry background, not so much from my horse experience.
It's a common misconception that electrolytes are something special. Really, its just salts dissolved in water. So as far as I know, anything that would increase salt and mineral intake should help.
Gatorade (if it was for horses) would actually work, to an extent. Gatorade is that is formulated so the salts go into your cells, "preparing" the cells for water intake. (This is why you're supposed to dilute Gatorade 1:1 with water, or take a sip of water for every sip of Gatorade.) The issue is that I don't believe it gives you minerals and salts that are necessarily good for you, just ones that easily diffuse into cells.
If other posts contradict this, ignore it completely.
Over the summer, my friends mare was colicing and we gave her some gatorade thru a syringe to give her some electrolytes until the vet got their (it was brutally hot out).
You can give the powder in their water, but if you do that, make sure you have another bucket of plain water in the event they don't like the way the electrolytes taste. That's how we usually do it, or use it thru a syringe. I used to give my boys sips of my gatorade...Toby loved the purple and red flavors. :P
Have you tried using heated buckets/troughs? They are great for keeping the water at a decent temperature that isnt too hot or too freezing that they don't want to drink...plus it keeps the work of knocking ice chunks down low. :)
It's got no sugar and smells like apple jolly rancher and my horse LOVES it. Its nice to have a strong smelling, delicious treat to mix in with his supplements whenever I have to include something like bute or antibiotics.
We've also used it with water in a syringe to help a horse (not mine) that was dehydrated and possibly colicing waiting for the vet.
I've been told that adding some epsom salt to their feed can do the same thing but the electrolytes aren't very expensive and serve multiple purposes explained above :)
I put a scoop of salt in Nelson's grain - everyday. The salt is in both his A.M and P.M feed.
When I ride hard - like am at an Event or Fox Hunt, I will mix Nelson's apple flavored Electrolytes with some water, and I will put it in a large seringe, and seringe it into his mouth. If I put it in his water, or in his grain..he wont touch it, so I have to put it in his system personally.