Red Filter for the Canon WPDC11 and WPDC21 housings (for the G7 and G9 cameras), which fits over the housing's port and can be installed and removed during the course of the dive.
The Red Filter is used to restore the red color absorbed by the water. In shallow depths, these filters can serve as an attractive alternative to artificial light sources.You set a manual white balance using your hand or other white area and that filters the cyan cast out.
Click here for further information regarding Red Filters.