I've chosen this shot of a sea turtle because there are already faint rays of sun streaking through the water behind him. 

Most of my underwater shots would not be amenable to adding rays of sun, because a lot of my photography is shot in deeper water.  This photo, however, was taken in shallow water, and there are very subtle streaks of sun rays shooting down behind the turtle, making this image a great candidate for adding or enhancing sun-ray effects, while maintaining a natural look.

Creating the File

• Open Adobe Photoshop® and create a new document (Go to File > New).  A square document at 5000 pixels by 5000 pixels at 72 ppi - any larger, and the brush preset function will not work.
  
• Ensure that the Background Contents are set to Black (click the colour box to set the colour manually).

Note:
It doesn't matter if the new file is RGB or Grayscale.

Creating a Brush Pattern
            
Select the Elliptical marquee (M) from the tool bar.  Notice that I have created the selection near the top of the file, to give the rays I am creating lots of room to spread down.  I have also created the elliptical selection quite wide, horizontally, in an effort to spread the rays across the image to some degree.
             
You may choose to create a perfect circle selection (hold the shift key down while dragging the elliptical marquee tool), if you require light rays coming from a smaller or more specific area, like light rays through a cave opening, for example.

Because the "sun rays" look a little choppy to me, I will run the radial blur a second time, using the exact same settings (this may be done easily by going to Filter, and clicking the function at the very top of the list.  This spot is reserved for the last filter function used in Photoshop, with the settings saved).  This time I have a much cleaner set of sun rays.

• Unsharp Mask will sharpen and define the rays.  Go to Filter > Sharpen > Unsharp Mask (Amount 500%, Radius 35, Threshold 4).
  

Creating the Brush

Now we are finally ready to create the brush. 

• Go to Image > Adjustments > Invert.
• Go to Edit > Define Brush Preset (give your brush a name).

Your brush should now be a part of your brush library.  Create a new layer on top of your image, and test your brush.  Choose the desired foreground colour (I use white).  If the effect is too strong, remember that the opacity of the layer may be reduced to soften the effect.

Enjoy, and happy diving!

This is a beautiful little children's book by Paul Mila (and published by Best Publishing Company www.bestpub.com), featuring his own, colourful underwater photography from Cozumel.  His story teaches the reader a little about the marine life (especially Harry the Hawksbill turtle), and weaves a sweet little tale about the importance of accepting friends' differences.  I love the message the story sends to the reader in a fun way, but also that it inherently promotes a love and awe for the amazing world that exists beneath the waves.

One of my favourite parts, is a page of questions at the end of the story (answers are provided on the opposite page) - is there a better way to interact and have fun with a child while they learn?  How much do they remember from the story?  How much do YOU remember?  Have fun with this together!  Awesome!

I will enjoy reading this story and marvelling over the underwater images with my grandchildren; it's something that I have missed doing with my own babes for many years now.  AND, your young reader can read to YOU!   A great tool for the budding reader to stretch his/her literary muscles (rated a level 3 for young readers)!

About the Author:

Paul Mila hails from Long Island, New York (although may be found more commonly in Cozumel), and left the corporate world to do what he loves best:  scuba diving and writing!  This is Paul's first children's book.  He has a great series of novels that have been in print for some time, and yes, the underlying theme of these thrillers is the ocean and scuba diving.

Check out Paul's thriller novels(great for vacation reading):