I've come across sweating / fluid / salt loss problems in (initially) well-hydrated people after as little as running eight miles with a heavy pack. I think (I'm not a doctor) that salt tablets on their own are frowned on now. We use an electrolyte powder. They will have different brands in Canada, but you get them from a pharmacy. What you're looking for is the rehydration powders given to children / adults when they've had diarhoea.

Cheers, Scott