Uninjured sailors are good swimmers and a life jacket is irrelevant when thrown overboard. If a sailor receives an injury and is unconscious then he will likely die if he doesn’t have on a life vest prior to being thrown overboard. The biggest threat to a sailors life when in the drink is water temperature and hypothermia.
Basic Training swimming requirements for the USN are just that basic.
Real Basic
3rd ClassSwim Test Pass Fail
If you have been thrown of the side of a ship that has just been rammed especially if you have been traumatized or banged up is a whole different story.
Third-Class Swim Test -
A third class swim test is a test to determine if a person can stay afloat and survive without the use of a personal Flotation Device (PFD) in open water long enough to be rescued in a man-overboard situation. The 3rd class swimmer qualification is the minimum entry-level requirement for all U.S. Navy Personnel.