A crossbow is not designed for underwater use because it cannot handle high density. The arrow is unable to glide through the water due to its immense mass per volume. It resists the arrow’s forward motion and slows it down before it reaches its intended target. Ask anyone who’s done underwater archery, and they’ll tell you how difficult it is. Even if the crossbow produces more energy than a compound bow, it’s still not enough to overcome water. You will still feel the power of the crossbow when you use it in water that’s 3-5 feet deep. Such water depths are not dense enough to slow down the arrow’s travel and affect its accuracy.