You can easily script such a system with the commands that Lemur V2 will provide you.
Here's a quick example:
:state=0:gvar1=500,state=1
:state=1,plrunderwater=1:state=2,entitytimerstart
:state=2,plrunderwater=0:gvar1=500,state=1
:state=2,entitytimergreater=1500:state=3,entitytimerstart
:state=3,plrunderwater=0:gvar1=500,state=1
:state=3,entitytimergreater=1000:incgvar1=-10,entitytimerstart
:state=3,gvar1less=0:addplrhealth=-10,gvar1=0
If you give that to an entity that is always active, it will literally control your character's "Air".
With the script above, the player will have 1.5 seconds of "free time" underwater where they don't lose any air value. After that it will begin to take off 10 Air units (Starting from 500) every second. Once the air value reaches 0, the player will lose 10 health after every second they remain underwater!
See how much power the user is given when using Water in Lemur V2?
The one and only,
~PlystirE~