The 50mm f1.2 is weather sealed, but requires a filter to complete the seal. Though, don't interpret that as water proof or built like a tank. I've owned a few and broken 2 of them due to harsh environments. If you're shooting in rain, high humidity, or dusty environments then I recommend using a clear/uv filter to protect the front element and a lens cover to protect the rest of the lens from the elements.
My first one broke while on a photo shoot in the Cascade Mountains during the winter. Unfortunately moisture somehow made its way into the lens (even with a filter on) and the frigid cold caused the mechanism to freeze up. When I turned the focus ring while the camera was on the lens wouldn't respond. I was unable to focus with it until I sent it in for repairs.
A second copy broke while doing a photo shoot on the beach, then transitioning to a speed boat. The harsh movements of the boat hitting the ways caused the magnetic focusing system to break and become unresponsive. From the what the Canon CPS tech told me over the phone, apparently the internal lens elements were dislodged and one of them cracked inside the barrel. It was too costly to repair so I bought another one.