Hi,
I have waterproof Pentax and bought neoprene case from an outdoor shop (the kind that can be used for mobile phone or sunglasses) - costs about $15. Cut a hole at the front for the lenses.
The case has straps at the back so all you need is to run some wide (5 cm) stretch material through. I bought that from material shop. Use velcro or clips to hold it together.
I suggest to run a safety line from your camera to your wettie or harness, they tend to fall off the head when you take a dive

The footage looks great, takes a while to work out the correct position on your forehead etc. I also used it for skiing.
And yes, you will look like an idiot with camera straped to your head

Also check out
http://www.helmetcamera.com/ if you want a proper camcorder set up.
Cheers
Jakub