Scubadiving sounds like a cool enough suggestion in itself. But it would have pretty limited use as only 3 of the maps so far have enough water for it to make sense (Aeolian Coast, Sainte-Anne and Wake (Insular Complex doesn't count for obvious reasons)).
And on those maps it's pretty easy to sneak in from the water already even without this (like in the villa and east farm bases on Aeolian Coast).
And for the record, you can't fire a M16 effectively underwater, or any other gun not made specially for the purpose for that matter.
True, you can shoot many of them underwater, but in most cases the bullet would not be effective past 3-5m at most, and it might severely damage the gun.
Better stick to your knife in water combat.
