filter damage drown
But the thing is, setting a damage filter shouldn't even be necessary insofar as I'm aware; none of the water textures should cause ANY damage AT ALL.
But I was asking how to negate damage, not how to deal damage 
the player still drowns in water, that's what I wanted to avoid, that's what I was able to do, that's what I can't do in Portal2 anymore.
my bad
I know, and I'm afraid I don't know how to help you, I'm just saying that it shouldn't be necessary, and, furthermore, why you need to do this. I'm sure there is SOME way of doing.
If you don't want the player to take damage, then just disable the trigger_hurt for that duration of time.
Alternatively, you could have a point_clientcommand that enables the "god" mode cheat.
[spoiler][SP] Alternate[/spoiler]
The point is, there IS no trigger_hurt, I believe.
ok, I'll update if I find a solution, thanks
There already is a solution.
Remove the func_water_analog, you don't need it. I don't even think it works in Portal. Its a Half Life series thing. Create a brush extending over the area where you want the slime. Apply a toolsnodraw texture to all surfaces, then one of the toxicslime textures to the top. Create another brush in the same location. Texture with toolstrigger. Choose Tie to Entity and select Trigger_hurt. Set Damage to 100. You will fall through water, so the basin needs another brush at its base. One way to destroy objects: place a trigger_multiple above the water, with the flag Physics Objects, and this output: OnStartTouch > !activator > Dissolve > 0.00 > No
To "negate" the damage disable the triggers when you want to ie; on button press.
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."
...There IS no trigger_hurt. He didn't add anything. He's talking about drowning, not hurt damage. It's still in the Portal engine to drown the player after being underwater for a time. I tried it out on myself a day ago.
WRY? this isnt a swimming game!
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."
I new to mapping. I don't know how hammer, or portal engine treats water exactly, but you could leave water only on surface. I mean the water to be only a few inches deep, after that a empty pool. You make the floor and the walls of the pool (or whatever you are using) to give the illusion of water. English is not my native language but I hope you understood.
Don't know if it is possible. If the engine treats water like in the real world, the water should drop, so it will not work for you.
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.