filter damage drown
Hey. Your problem is that the player is dying in water.
I tell you: Even if you don't have a trigger_hurt, you need one to keep the player alive!
Just set the damage to -100 and the player gets healed 100 points every half second.
The !player SetDamageFilter method have worked for me in other games.
"I hear voices. But I ignore them and I just carry on killing."
You can't even see underwater in Portal 2. It isnt even water it is toxic sludge.
PortalStories.com
"Oh, in case you got covered in that repulsion gel, here's some advice the lab boys gave me: DO NOT get covered in the repulsion gel."
To whom you are addressing? That's way I proposed only a layer of slime.
PS. Personally I don't like "athletic maps" were when you don't make it you die or you lose a lot of time. But that's just me.
Both the methods I knew don't work in Portal2 so I resolved otherwise.
@hyperion1is: I need the player being able to swim
@Skotty: good one, but to maintain the trick the player can't be hit by turrets or lasers.
@iWork925: with custom shaders you can
Thanks guys.
Another thing how you could use the trigger_hurt healing method:
Count the time a fully healed player needs until he's drown (without any healing of course). Now just set the trigger_hurts damage value to a fitting.
Just use your time. 100/t (t is your time) and this /2, because trigger_hurt is (if I remember right) doing damage twice a second.
So the trigger_hurt will just compensate the drown-damage but not the turret damage.
If Portal 2's autoheal is disabled while drowning, just set your healing value a bit higher.
Oh another hint: In Coop, the player get's a drowning animation and he can't do anything. There any heal is senseless. If there is no hurt, the player never gets out of this animation.
@skotty: it makes as sense as using setdamagefilter or playerproxy... it doesn't work even if it should 
With a trigger_hurt and dmg= -99999999 the player still drowns or at least the screen starts flashing red.
Clearly it's built-in, like the fact that you can't fire portals underwater.
So, there is just the possibility to make real fake water.
For that I would try a func_illusionary. If the water won't render, a change on it's VMT would follow for this use.
With a trigger I would enable and disable something like a overlay with a animated water texture, so the screen looks like underwater. The sounds could be damped by using soundscapes with a special DSP. Water splashs are harder to do. If it's needed, I would create a lot of triggers and a lot of env_water_splashs.
Because of this complexitiy I would, if it would be my map, change the whole map or cancel it.
You can swim in sludge for a second and as long as the edge of the slime pool is only 16 units you can swim up and out.
[spoiler][SP] Alternate[/spoiler]