Here's a fun excerpt from a TV program showing how water, so much more dense than air, can slow down a bullet.
I used to think that the stories of divers attacking harbors, and swimming safely underneath a hail of bullets fired by sentries on shore, were just that - stories. However, way back when, I saw some South African divers training at Walvis Bay prior to an operation in Angola. Part of the training was to show them that sufficient depth of water (about 3 meters, or 10 feet) would, indeed, protect them from AK-47 fire. Sure enough, as they swam along next to the quay, their instructor fired an AK-47 at them, and the bullets were slowed to a crawl before they could reach the swimmers. It was very educational.
(Note to readers: DON'T try that at home - or anywhere else!)