What is the piece of white filter in front of the flash for?

Hi, very new to U/W potography, need some advise.

may i know what is the piece of white filter in front of the internal flash for?

when do i put it on and when do i remove it?

if in U/W, any tips on how can i keep it when removing it from the housing without losing it?

the filter is called a diffuser, it spreads the light from the flash around so you dont get so much back scatter, and in some housings so you dont get reflections from inside the housing itself. I would keep it on most of the time.

You should use this white piece of plastic in front of the flash if you're using the flash in the water. It diffuses the flash so that its less likely to pick up on the particles in the water. This is known as backscatter.

If I was you, install the diffuser and then leave it in the housing.

I try not to use the internal flash. Ideally you need an external flash.

Personally I like using a compact camera in my BCD pocket and I prefer the look of natural light shots (no flash) with a red filter to balance the colours. I dive only tropical locations so can get away with it.

Very simple beginners guide here.
http://www.zerovisibility.co.uk/photography.htm

Sample Red Sea images:
http://www.zerovisibility.co.uk/Gallery/Egypt1/index.html
(Almost no digital enhancement either...)