Kristy just told me about that effect today, by PM, here's what she said.
="Kristy"]In the layers palate, click the add a layer style button
Then choose Inner Glow
Next change the blend mode to normal
Then double click on the color box and choose black (if you want black, you can also do white for a diffused border, pretty for your beach shot)
Then you mess with the choke, size and range slider until you get what you like.

