Any bike specific GPS is already waterproof. The one on the left in Deej's picture is a 60csx or 62s which are waterproof. The one on the right looks like a Nuvi. I think some of those are waterproof, but most aren't.

 

Nothing beats the RAM mounts, I have them on everything I have ever owned, its nice that once you have the mounts on everything, you can take the other components and use they on any other vehicle you want, move things around, etc.
I use them for my GPS, iPhone, and cameras.
~John

 

I know my 62 is waterproof. Not sure on the flat screen styles. I know some are waterproof from the factory.

 

Deej- Do you have the short arm or the long arms? Also I can't tell from the pic, are the arms metal or plastic?

 

The arm i got appears to be aluminum coated with a thin layer of rubber to prevent the ball from spinning in the mount. stainless hardware. Good quality stuff.

 

This is what I was thinking of.. not gps.. but can be modified easily..
HOWTO: Cheap Camera Handlebar Mount