The 5.66 is an odd round, it has a long dart rather than a regular bullet. But the cartrige itself is designed for underwater combat. It is used by the Russian APS and ASM-DT amphibious assault rifles. The Projectile is hydrodynamic unlike regular bullets which would stop at around 3 feet in water. But the 5.66x39mm ammunition would go through the water with high velocity and with deadly accuracy.
Although the 5.66x39mm ammunition is extremely accurate and deadly underwater its ability to fire out of water accuracy is extemely limited to close range.