Cubemaps
Dont forget the search feature.
http://forums.thinking.withportals.com/post80840.html?hilit=cubemaps%20cubemaps%20crash#p80840
In case people come across this thread in the future: put your bsp in the most recent portal2_dlc folder before you build cubemaps. That should fix it.
Also, the game's resolution has to be above 800x600 IIRC.
[spoiler]WOM Test 1
Laser Cube Quest
Mho' Power - Community Spotlight!
Four Corners[/spoiler]
I think it's more useful to directly make hammer to save a copy of your map in the portal2_dlc2/maps folder:
This is how to build cubemaps after the map is there and you have loaded it:
1. Open the console and introduce buildcubmaps
2. As by default the game is running on HDR, now you need to build the LDR cubemaps, so enter mat_hdr_level 0
3. Now enter buildcubmaps again (these are for LDR).
4. Then mat_hdr_level 2 (back to HDR again)
useful tools and stuff here on TWP 
[spoiler]
[/spoiler]
Also, your goo might look terrible if you've done any compile less than final. Make sure that's not what's causing your goo troubles.
Talking of goo, well, you should add a cubemap for each 'independent' goo area and assign the cubemap to it by going to the "brush faces" property, and hiting the button "Pick" and selecting them (wiki reference); and holding Ctrl key to select multiple brush faces that makes a whole water area:
Then, since that cubemap is assigned to the water, you should need to add another one more for the reflections in the rest of that room 
useful tools and stuff here on TWP [spoiler]
[/spoiler]