If you fail to neutralise the blue in your underwater photography you will end up with amateurish photos that have a horrible cast to them where the whole image is a blue mess. The best way to achieve this neutralisation is to manually white balance the camera and use a red gel. I use the a red gel which give me consistent, stella results.
When using red gel (use a blue gel if diving in green water) there are many ways to attach it to the lens. Some lenses have provison to front mount filters and while have some holders at the rear of the lenes allowing you to rear mount it. I have used lenses that have neither front or rear mount options. My work around for this is to cut the gel to the size of the rear element and to simply sellotape it in place. It works perfectly.
You will need to manually white balance you camera every 4 or 5 meters anytime you change depth. I like to save a white balance setting for 3 meters depth the as I enter the water simply select that setting. I can return to that setting as I ascend or on a safety etc. This is particularly handy when you ascend in the blue and there is nothing to use a white balance reference. On the subject of using a reference for your white balance, I have used everything from a grey card, a white t-shirt tied around my thigh, air tanks, my hand etc etc.
One of the most effective things I have found to use is off-white coral sand or coral itself. It really is trial and error and somethings work better than other at differing depths. When you get it right, you’ll know as your monitor with burst into life with rich vibrance and crisp contrast.