Can I soak my helmet in water? I have a lot of sweat in all straps and padding in my helmet. 

When i ride in heavy rain the salt from my sweat starts dripping down my face. Its very unpleasant especially if it goes in my mouth. 

How can I get rid of this salt?
The helmet itself is (probably) polysterene foam inner and a harder plastic shell, it's fine to wash this in water and some mild detergent (eg dish soap)

You can try and wash the straps in hot soapy water.

The problem is the pads. Usually washing the pads means that they come unglued, the surface layers separate from the inner cushion part. All you can do is keep the extra ones you get to size a new helmet and swap them.

I don't know if pro racing type helmets come with multiple sets of replaceable straps and pads - but they should.