I'm missing the Serpent Icon to get that imortant book.
I know its supposed to be somewhere in/under the snake pens, but when I swim under there I can only get from one pen to the other and up a narrow tunnel where my party gets stuck and drowns even though it looks like their heads are above water.
I'm in need of help!!!!!!!!!!!!!!!

you can only resurface at the point were you entered the pen so make use of the map facity of your character. Some have also had problems getting through the tunnel to the area were the idol is so select the character with the beast atheletics rating as lead and look down and forward to get through

You must pull the lever inside one of the Serpent Hold rooms. For some reason, when I didn't do this, I could not swim past that point you mention and also started drowning. Once I reloaded my game and pulled that lever, that bottleneck seemed to disappear. Basically, just pull that lever. It's on the left when you enter the serpent hold room (the left one).