Hi dude...
What you need to do is...
Enter the room with water and pipes, go under in the water, now climb the ladder untill you reach the end of the ladder (wich is where you came from).
Now you'll see that you can jump on the pipes, on a vertical pipe is a sort of switch (it has the shape of a circle), you'll reach it from above, falling from one pipe to the other...
When you turn the switch the level of the water will rise, it will go higher, now go in the other room, you'll see that now you can just swim through the hole in the wall...
I think you will be able to find your way out now...
if you have anymore questions about this game please go to the forums, I've started a topic there, it's called Half-Life 2, here's the link:
http://forums.supercheats.com/forums.htm?forum=7&
Good Luck...