"The basics: 18 soldiers did a competitive 25 km march (taking about four hours), carrying 26 kg packs and wearing full battle dress, in temperatures averaging 40.2 C and reaching a high of 44.3 C (112 F). They were allowed to drink only water...."
"At this sweat sodium concentration, average total sweat sodium losses during the march could have been >240 mmol. Yet despite such large losses that were not replaced during exercise, participants maintained their serum sodium concentration. This confirms the now well-established finding that serum sodium concentration can be maintained during exercise without the need for acute sodium replacement during exercise."
As Neal Stephenson put it, you're a three-billion-year-old "nightmarishly lethal, memetically programmed death machine". Do you really think you're going to be stopped by a temporary shortage of salt or water? Please.
Read the whole thing.