Every diver has noticed that the underwater environment is one of monochromatic hues rather than distinct colors. Depending on your location, objects will take on a blue or green cast at the expense of all things yellow, orange, red, etc. This is because water acts as a filter of red light. The deeper you dive the more the red spectrum is filtered from the ambient light. You can however, emphasize the existing red light by filtering out the blue spectrum with a red filter. The most important thing to understand about red filters is when to use them and when to use lights to restore color. In the dark and murky waters of Monterey, I use lights to restore color to my video. This is because ambient light does not penetrate very far into the water column. But in tropical locations, where the visibility can be 100' plus and sunlight is abundant, I'll use the red filter in every situation but very deep or very shallow. How deep or how shallow is dependant on your location.
A good example of improper white balancing while using a red filter.
At or near the surface a red filter will give your scene a noticeable red hue. If you have a housing that allows you to manually white balance you can correct for this red hue by white balancing through the filter. If you do not have a housing that can white balance do not use the red filter until you get deeper and more of the ambient light has been filtered out. Or just correct the red color in post-production when you edit.
Many housings come with an internal flip down filter. If your housing does not have this feature you can purchase a UR Pro filter that will screw onto your cameras lens. These come in two varieties: one for green water that have a magenta cast, and one for blue water with an orange cast. Most people use the Blue water filter as this will work well in most situations.