Drill instructors are responsible for the health and well-being of their recruits. There are certain things that drill instructors cannot deny recruits. Two of them are head (bathroom) calls and water breaks. One of the main themes you will see in boot camp is the insistence of drill instructors to ensure that recruits are drinking enough water. What goes in must come out.
Every recruit is required to carry at least one canteen at all times. Most of the time the will carry two. There will be times when the platoon is made to line up and drink their canteens until they are empty. The drill instructors will make the recruits tip the canteens upside down over their heads to show that their canteen is empty. Failure to drink enough water during boot camp can result in a recruit becoming a heat case.
A heat case is when a recruit is overheated or dehydrated due to lack of water. The body cannot cool down due to the lack of water and the organs will begin to shut down if the recruit’s body is not cooled down. This is a very serious matter and drill instructors take it very seriously. Not only can it lead to sickness and in extreme cases death, but it delays training when all the recruit had to do was drink more water.
Drill instructors will warn you in the beginning of training about the dreaded and legendary “silver bullet” when a recruit becomes a heat case. The silver bullet is not a myth. It is a reality and the unfortunate future of any recruit who does not listen to drill instructors and fails to drink enough water. Basically what happens when a recruit becomes a heat case is that a corpsman will come over to the recruit and regardless of who is around (other platoons, persons of the opposite sex), pull the recruit’s pants down and insert a silver metal thermometer into their rectum. The purpose of this is to obtain a core temperature in order to assess the seriousness of the heat case. Time is of the essence which is why it is done quickly and on the spot.
Drill instructors do not seek to embarrass the recruit but this becomes a matter of life or possible death so all notions of where they are no longer matter. They will then take them away to medical after wrapping them in a cold sheet. They keep two coolers full of ice with the platoon pretty much at all times in order to have it on hand in case a recruit falls out. Inside the cooler are sheets that are ice cold and can assist in helping a recruit who suffers heat stroke.
The absolute best advice is not to become a heat case. Drink plenty of water.
Also, do not ask for a head call unless you need one. There are recruits who will figure out that drill instructors cannot refuse them a head call and will constantly ask for one as a means to get out of the hot sun or training. Repeat offenders will soon be spotted. It is a real jerk move as you are essentially thinking only about yourself instead of realizing that your entire platoon is out in the hot sun together. In boot camp, it is always about the platoon, not individual desires. If you are unwilling to sacrifice your personal desires for the good of the group then you have no business being in the Marine Corps. The only way the Marine Corps works is when Marines are ready to fight and die for the brother or sister beside him or her.
Source: Personal experience