Okay, a few things caused problems so far. As I thought, visible in reflections and refractions weren't check on for the sphere and plane. The settings on the mia_exposure_simple and the framebuffer were screwing you over big time. I put them back to default because everything was pretty dark. Also the "scale" attribute on the ocean shader determines whether the water looks like you're viewing an ocean from a airplane or sitting in the kiddy pool. So taking it down to 0.1 helped with the transparency. I was getting some nasty artifacts on the ocean shader, until I changed the Directional light to an Area Light :s. Dunno what that one was about really.