This is absolutely fantastic !

I wanted to address pedophilic vibe some might get from movie , it wasn't that though there were no apparent sexual undertones in this. It was rather more of motherly/protective/innocent nature of Stella.She was unaware of Nicolas's inability to survive under water. His resistance seemed like his constant struggle to get out to break free. The breathtaking scene (pun intended) where she uses her mouth to help him breathe and experience what comes naturally to her exploring sea.

Starfish doesn't appear to be symbol or metaphor just imagery which changes it's form throughout. Nicolas is repulsed by it , and all women seems to be some primitive species and he is in that transitory phase where he can't comprehend as he's evolved into something else. So both Stella and Nicolas are oblivious to what it all means(and so are us) but are still helping each other to survive , finding their own paths. The ending seems rather disappointing , all can be seen as weird medical experiments.It could very well be co-existence of different species/primitive beings as nothing is laid out or fed to you.