It should come as no surprise to anyone that smoking is one of the most detrimental ways that you could affect your health.  Tobacco use affects the body in serious ways by withholding essential nutrients from giving your body the boost that it needs to perform at the highest possible levels.  If you are considering training for the Navy SEALs, smoking is one of the last things that you will want to do as it will bring your performance down exponentially. 
Not only will the nutrients which are going to strengthen your body become blocked, but your breathing will become labored as well.  The more that you smoke, the more deadened the tissue in your lungs will become.  It will be harder and harder for you to inhale properly and when you do take a breath, you will not be able to absorb the full levels of oxygen from it.  That reduced oxygen intake will lower your ability for running, swimming, heavy lifting, and even with concentration.