What you want to see? - The Oxygen System addon be added to the server.
Why should we add it? - It would deter/punish players (in the few cases that it does happen) for hiding underwater on the surface for an unrealistic amount of time.
What are the advantages of having this? - Realism, would stop/punish people from hiding underwater for large amounts of time.
Who is it mainly for? - Anyone on the Surface.
Links to any content - https://steamcommunity.com/sharedfiles/filedetails/?id=2115403509
I tested this myself, after about 10 seconds, the player who is underwater will start taking 20 damage a second. The amount of oxygen, level to start drowning, and it's HUD can be edited in the config. You can also exclude classes from drowning.