I'm going for a slightly different underwater theme with my game, as I thought it would offer a nice change to the usual space ships. I wanted to do a swirly background effect to mimic water. I guessed I could do it with shaders and after a bit of googling I found a suitable effect. I dropped it into my game and sure enough it looks quite nice!
All I needed to do now was to send it into the background layer, and expand it out to fill the screen.
Looks OK and was exactly what I wanted. Shaders are really powerful and you can do some amazing effects with them, I highly recommend having a look if you need to add a bit extra to your games.