Red is lost from the colour spectrum rapidly as you descend through the water column. To counteract that most cameras will allow you to manually reset the white balance (effectively resetting what the camera understands as white) which will bring back a lot of red to the image. I tend to use some white sand or my hand to take the white balance reading but others use a white slate. Ideally you should do this every time you descend or ascend a few metres but there is some leeway and you don’t want to miss shots because you are desperately trying to set the WB! If your camera doesn’t have this function you can choose auto white balance which will work well until you move the camera when it might change during the shot (not nice) or you may be able to dial in approximate WB in kelvin.