Brush vanishes under water

Edit 1: Kindly ignore the leak in the first picture. I enclosed the map in a box for testing purposes, so no leaks.

I can't explain it. Just a little below the surface, everything is fine. Then I swim down to the bottom and it's like the brush turned invisible. Any help is much appreciated.
http://i249.photobucket.com/albums/gg204/Immortal-D/tf2error1.jpg
http://i249.photobucket.com/albums/gg204/Immortal-D/tf2error2.jpg

bob+M|M+ said:

hmm weird error. I had a similar problem when my water levels were uneven.

Click to expand...

I do have 2 different water heights right next to each other. But even if that was the cause, why would that particular brush disappear? I'll have to do some experimenting tonight. But not right now, cuz Samurai Jack is on