I think it would make underwater combat slightly better if you would have the ability to reload you SDAR rifle, while simultaneously moving underwater. Since having to stop to reload underwater makes you an extremely easy target considering there might not be much cover if you're close to surface in deep water.

On another note i think it would be reasonable to allow using chemlights, IR Grenades and your inventory overall while underwater. Perhaps an ability to attach a chemlight to your body or hold it in your hand to light the way seeing that divers do not spawn with NVG's by default.

I do not know how NVG's would perform while submerged but atleast chemlights should work or maybe you could make it possible to attach a flashlight to the SDAR rifle.

As of right know its quite impossible to see underwater at nighttime without an SDV or NVG's.