Portal Chambers under Water
Maybe this could work if you combine it with pestchamber's Aperture Science Cleansing Chemicals idea (in his Nevada map). You could make part of the map underwater, and you need to find a barrel of ASCC to make the water safe before you enter it. Or use the clear water from the Logic Portals map. I think that an underwater map would be both epic and a way to stick it to GLaDOS. ("Do not submerge The Device in water, even blah blah blah" lol)
In fact, the chemical cleansers thing is proof that GLaDOS lied about you being electrocuted by putting The Device in water...
On the other hand, the underwater effect would be a real pain to render for people with graphics FailCards. But it's definitely possible. Hard, but possible.
*fifty cakes fall out*
Time out for a second. That wasn't supposed to happen.
um. video games have been doing underwater rendering since ATLEAST Quake1 (maybe sooner (don't feel like searching)) so this is not true in the least. only time it gets hard on video cards is if its multi-faced water (like a 3d water surface) and/or you can see through the water, but thats not "THAT" true, cuz quake2 did that even without 3dfx compatible cards.
Edit: WTF am i thinking, Duke3D was (probly) the first FPS with underwater sections.
If the water can't be seen through, then what's the point of that? Since my GeForce card with DX9 won't render either shiny water OR the ripples inside an unconnected portal 
, then you'd need a good card to render water reflections in an underwater chamber. And since I will NEVER go to Vista (Microsoft's biggest mistake), the highest I can go is DX9, and a 256MB card should be good enough, but apparently it isn't. Stupid computer...
*fifty cakes fall out*
Time out for a second. That wasn't supposed to happen.
BioShock made the player fell that he/she was underwater.. but you were not. The effects are in the windows and the outside is a green skybox and the buildings are a type of brush/model. It is not using the source engine so I don't know for sure..
The more you give, the less appreciated it will be received.
holy crap. first post in wrong thread. thx for making me verify. i am a bot tho. srsly.