The game is fairly nonlinear, so you may not have done something that is a "trigger" for the game allowing you into the bathyscaphe.
Just a guess: have you broken the code on Linda Manchek's locker? That gives you the code for bathyscaphe #2.
Also, has Lea already changed into her bathyscaphe suit?
EDIT: You also might want to open up and read Mancheks' file -- it has instructions the game may insist that you read.

Hope this helps.