Using a polarizer/polarizing filter
The photo shows the differences between the use of a polarizing filter and the image with in the absence of a polarizing filter.
What is a polarizer?
To be more technical, light becomes polarized when reflecting off of a surface so that means all reflected light off of non metallic surfaces like water is reflected at a single plane. The polarizer only lets light through at a single angle so by rotating the polarizing filter 90 degrees, we can eliminate all or most of the reflected light by creating perpendicular planes at which the reflected light can’t go through.
When photographing water surfaces such as the Caribbean waters where the green/blue colors really create an image, using a polarizing filter can eliminate the sun and allow more of the color to come through in an image, otherwise hidden by the blue reflection of the sky.
Polarizing filters vary in price but are primarily quite cheap and although the lower quality filters may degrade images slightly, the value added by eliminating unwanted light is worth it. Using a circular polarizing filter (one that you can screw onto your lens thread mount) is a quick and easy way to improve your images.
How do I use a polarizer?
To use it, simply screw on the filter to the front of your lens, making sure the thread size matches the filter you purchase (not to be confused with focal length). Nikon kit lenses are 52mm and Canon is 58mm (you can find these numbers on the lens cap or on the front of your lens) but you can buy step up rings to be able to use a 58mm filter on a 52mm lens which is what I recommend so you have a bigger flexibility in the future (you can screw larger filters on smaller lenses but not otherwise). You should be able to turn part of the filter around 360 degrees AFTER the filter is on tightly. Don’t overtighten your filters though!
The image test shown was done using a Rocketfish 58mm CPL: $20 at Best Buy or Amazon.
For non DSLRs, you can achieve the same thing by manually holding these filters in front of the lens directly against the glass of the lens. Make sure your filter is larger than your lens glass element. Notice how this technique can be applied to iPhones, point and shoots and anything else that you can use to take photos.