Being underwater in that case will give you some protection, but not absolute protection. If the crash started in the water with you (. The Build team filled the doors of several cars with phone books and fired several firearms at the vehicle. The phone books were able to stop 9 mm. However, the more powerful deer bullet-shaped shotgun bullets and the M14 assault rifle could easily pierce the door and phone books.
Since the telephone directories could not stop all the bullets, it was declared that the myth had been destroyed. However, the construction team continued with the experiment to see how many telephone directories would be needed behind the door to stop a bullet shot from a deer and discovered that only two phone books were needed. No, being underwater won't save you from a grenade explosion. The explosion will cause the same damage to your body, whether you are on land or underwater.
Because water is much denser than air, with a high surface tension, acoustic waves tend to bounce rather than enter (that's why you only hear things underwater while you swim), and intense wind tends to create foam on the surface instead of causing turbulence underwater. While gasoline could only break the nearest disc above the water, ammonium nitrate broke all discs and dynamite broke all discs, except for underwater discs located 15 meters away. However, if a target is underwater, the chances of dodging a projectile impact are 100%, since the drag force in the water is so high that the projectiles would disintegrate due to friction before they could cover the 5 meters. The way I see it is that the water will protect me from the heat, so I can come to the surface after the explosion and escape.
Because water holds neutrons so well, the concentration of radioactive isotopes would have to be high enough for radioactive fallout to be as dangerous as airborne fallout, but be careful not to swallow too much. For comparison, let's imagine that the target is two human beings who are 5 meters away from two grenades that are about to explode: one underwater and the other on land. The whole premise of this debate revolves around the fact that someday you'll have to survive a grenade explosion and the only thing you can choose is to dive underwater or try luck on land.