Question: Tom Hardy puts his shotgun shells in his mouth before swimming from his patrol boat to the larger boat. Why is this?
Answer: Because they'd be completely useless if he allowed them to get soaked by the water.
But his head comes out of the water. So the shells got wet anyway.
Answer: That's not true but the shotgun shells he was using are waterproof.
Answer: Because Hollywood. Most ammo is perfectly fine after getting dunked underwater. I had a bucket full of shells filled with snow, and I didn't find them until springtime. They worked just fine. Most people don't know that, though. Evidently, neither do movie makers.