Any ideas?

Google is your freind.

Edit: If the water never shows up it's probably either the texture, or Portal 2 dosen't support it at all.

gf1024 wrote:
If your map is not extremely huge and complex, you may want to try, if all else fails, instead of raising the water, lowering the rest of the map...

That's how the Pros crash they're maps! Really guys It's not worth it.

edit - sorry missed that youtube link above

http://developer.valvesoftware.com/wiki/Moving_Water

Quote:
Though moving water is possible, it has some limitations.

The water shader cannot be used. However, bump mapping is allowed (akin to "cheap" water).
Moving water will not always react the same as normal water. Client-side entities, such as broken props in HL2MP or ragdolls, do not float in this type of water.
Splashes (most noticeably those caused by explosions) will not render correctly. Splashes appear to be created at the water's original position, rather than its current position.
Underwater fogging does not occur, making the water seem "bright" compared to normal water brushes.

There's a workaround to the fogging - see the hl2 airboat level maps. & cheap water can look ok if you give it a decent envmap.

In portal all water is poisonous so you don't really need it to act like water, like in other source games.