Maybe the tub is wider than we think, due to foreshortening, and there's a third person behind Cassie who's laying on one side, propped up on an elbow, sort of fitting herself between the other two and the side of the tub? And the tub slants down, so it's very shallow on the far right end and that girl's head is still above water, while it's very deep on the far left end, allowing room for the blonde to be kneeling.

:/