If the terms 'violent' and 'masturbation' doesn't set up an image already, just imagine how you might look for your lost keys, but sexually, and on 4Chan.
Granted, my mental catalogue of masturbation scenes isn't exhaustive, but the aggressive nature alone speaks for itself. This eventually leads to the most violent masturbation scene I've ever seen. He imagines the mermaid throughout the film, fantasizing about her and what she might look like. Early on in the film, Pattinson's character finds a mermaid figurine in his bed, nestled in a hole about midway down his mattress.