In the Age of Sail, chains were not shot at personnel, they were shot at rigging. Snipers were only effective at close range with their inaccurate muskets, while naval cannon could penetrate oak at distances over a mile and produced clouds of splinters that traveled for 40 yards. Explosive shot and fire shot were rarely used. The vast majority of all casualties in gunnery battles (such as most fleet engagements) were caused by shrapnel, and that should be self-evident.
Look at that video. That's not mythbusters you're looking at, that is a real gun and a real ship. No weapon of the period can compare to those clouds of splinters. They are really are interior grapeshot. You would need a modern grenade to match the effect. Towards the end there is a piece of wood only four inches long that penetrated a plywood dummy. Imagine what that would do to flesh: a casualty for sure. Lethality isn't almost never the point in combat, and in any case, getting sprayed with a dozen pieces of shrapnel is more deadly than a soft, low velocity musket ball any day.
This is how damage was done to men on the gun deck. They were protected from small arms and grapeshot pretty well, so roundshot would be relied on. Direct hits would dismount cannon and splinter rigging, but the splinters were the main issue. The gun in the video is a 32 pound carronade, firing a ball 6 inches in diameter. A smaller ball, such as 6-pounder, may have difficulty piercing a warship's hull, but it would still cause splinters, especially if it hit something above decks, like a mast. As seen in the video, small splinters can be dangerous, but with guns that small you wouldn't hope to do much damage. Guns as small as 9 and 12 pounders were used in frigate engagements which could be very lethal.