The way I interpreted it was that Rennie had experienced a traumatic event in Crystal Lake, similar to Jason, they were both drowning, and both sort of betrayed by their supervisors. So Rennie got some of the curse of Crystal Lake and a connection to Jason. She can see the innocent boy (his soul) needing to be released/saved inside the curse that is the serial-killer Jason.
So she sees his fully non-deformed body/soul at first in the window calling for help.
As Rennie becomes more distressed due to the situation she is in, the boy, Jason, appears to get more and more deformed. It's like the initial vision (in the cabin window) was how his soul looks (no defects) but the more and more she had related escalated trauma, she started losing the innocent soul which transformed into the ugly vision he became or how people saw him (also how she saw him in the lake because her uncle told her a terrifying tale about Jason and that he is to be feared, so she saw him as others did, as a scary monster).
Once Rennie defeated the evil shell of Jason that kills (defeating the curse, if you will, and getting over her fear of water), she freed Jason (the boy), saving his soul, and her own, from the curse.