Charles Auge and Edwin Saez had landed at a canal junction at dawn last Thursday as part of a major US offensive against Islamist insurgents in the key province of Helmand.
They were engaged in an intense battle through the heat of day against dozens of gunmen who were determined not to lose control of the Mian Poshtey intersection in the south of Garmsir district.
When their two-and-a-half-litre (five-US-pint) water backpacks ran out, Auge and Saez looked to restock from the bottles that Echo company from the 2/8 infantry battalion had brought with them on the helicopter assault.
But as the company came under constant fire, the supplies were limited and the water scorchingly hot when it did arrive.
"We were on the flank beside a thick grass berm, and in the middle of the day the sun was so strong and there was no shade," Auge, 24, said. "I began to feel dizzy and everything turned white."
Saez, 21, also became a "heat casualty" soon after, having shot at, and apparently killed, two gunmen who were firing at the Marines from behind a wall.
"I started slipping in and out of consciousness," he said. "The water we got was so hot it burnt in my throat."
The two Marines became so seriously ill that they were evacuated from the battlefield by Red Cross helicopters that came in under hostile fire.
They were treated with intravenous drips and ice baths, and kept under observation at a field hospital for three days before being released, now recovering from the ordeal.
One of their fellow Marines was shot dead in the fighting last Thursday, and they say a total of five men were flown out of the battle after becoming dangerously overheated.
With temperatures in Helmand reaching 115 degrees Fahrenheit (46 Celsius) in the past week, soldiers face huge problems staying hydrated as they fight in helmets and flak jackets, loaded down with weapons, ammunition and water.
Basic Marine combat kit weighs a minimum of 50 pounds (23 kilograms), officers say, and the men fighting in Mian Poshtey were carrying hundreds of extra ammunition rounds each as they did not know when they would be re-supplied.
The terrain in Helmand varies from the dusty plains of the desert to the river valley, which is criss-crossed with ditches and walls.
Both provide a brutal environment for US troops despite extensive conditioning before they arrive in Afghanistan.
Active soldiers can get through 10 litres of water a day and providing water at the frontline is a major logistics priority, according to Captain Micah Caskey.
"A heat stroke can be fatal, and the heat definitely claims more victims among Marines than the Taliban.
"The balance is between the protection provided by a larger plate carrier (flak jacket) and the problems of heat and mobility."
Despite their experiences, and with temperatures forecast to rise further, Auge and Saez said they were anxious rejoin their fellow Marines in south Garmsir as soon as possible.