Using the SDAR works fine in water. What doesn't function is the use of other weapons at the surface of the water when floating with head out of water. If wearing a Rebreather, soldier will carry primary weapon. If wearing other clothes, soldier will sling weapon to his back. There is no access to Gear at any time while in water, no ability to switch to pistol, and aside from the SDAR, there is no way to fire any weapon while in water floating on the surface.
This inability ruins the ability to swim/dive with any weapon other than the unsuppressable SDAR. Maritime Operations requires a soldier to be able to fire his weapon from water when at the surface.