Why not have the option to carry an underwater pistol, like the HK P11? In real life, special operations units can carry their "normal" weapon such as the m4 or mp5 on their back when diving, for use on land. In game, just like real life, the underwater rifle is inaccurate on land, and using STANAG's only helps a bit.
If we had the option of a weapon such as the p11 for underwater use and then a "normal" gun for land use it would be great. A dive knife would also be a useful addition, as well as underwater explosives and mines.
Although some of these next things have already been mentioned, we should also have the capability to reload while swimming and shoot whole moving, although the shooting should be inaccurate (just like standing/fatigue). I have been caught reloading by enemy divers, even though in real life you would be able to move to get out of the way - you won't reload nearly as quick, but at least you wouldn't die.
We also need dive lights and glowsticks, both for the player and the option to colour code teams through the command menu and have the divers use chemlights specific to that assigned colour.
This has been mentioned, but we should get NVG's for the divers, at least in inventory for land use. You could even create another type of diver, so you would have divers w/ and w/o NVG's in the editor. I do think it should be standard that they have NVG's at least for land, but there are water-useable NVG's in real life, but they are limited to depth (another easy thing to implement, just have them start to black out when you go below ~20-30 m).