When Susanna considers her therapist's point of view regarding her condition in chapter 10 (and how it may have led to her admittance to McLean), she imagines his perception of her using the following metaphor:
It’s a mean world out there, as Lisa would say. He can’t in good conscience send her back into it, to become flotsam on the subsocietal tide that washes up now and then in his office, depositing others like her. A form of preventive medicine.
Susanna has a complicated relationship with her time at McLean. For much of the book, she resists the idea that she was supposed to be there, questioning what "crazy" even means. Often, she feels both rejected by society and completely helpless against her feeling of isolation.
Comparing herself to "flotsam" on a "subsocietal tide" when imagining herself through her therapist's eyes demonstrates how she feels dehumanized by the people around her. She feels he and others perceive her to be as worthless and inert as ocean detritus—a single speck on a tide of other so-called "crazy" people. She also feels like she has no autonomy and is at the whims of her physicians, much like flotsam is carried in and out with the tide. This metaphor is Kaysen's way of critiquing a mental health care system that was, from her perspective, stifling and unconcerned with her humanity.