DSG Technology of Cyprus/Norway have developed a supercavitating underwater round which can be adapted for standard rifles, allowing Special Forces divers to fight both above and below the surface. I wasn't aware of this until someone pointed me their way today. Like all good ideas it seems so obvious in hindsight, it just makes sense.
Check out their website: DSG Technology
Historically underwater weapons have been cumbersome, expensive and rarely needed. In fact underwater guns have possibly never been used in combat - they are a last ditch option. Modern rifles are ever more comfortable in the water (limitations accepted!) so combining the above and below surface weapon into one seems an obvious evolution, especially as it allows a seemless transition from underwater to above water defense for divers emerging from the surf for an over-the-beach mission.
Multi-Environment Ammunition (MEA) is effective as a standard round but can be fired into or out of water, even at a low trajectory of just 2 degrees. As the round moves through the water at over 100 meters per second the shape creates an air bubble (supercavitation) allowing it to move with minimum drag.
Underwater range for rifle ammunition is up to 11m for 5.56mm and 22m for 7.62mm rounds which compares to dedicated underwater weapons and fits with the poor viibility experienced in covert ops.