Does keratin hair treatment prevent water from reaching my actual hair when i wash it?? does it form a layer around each hair and do so?

Not exactly: Contrary to what it sounds like, a "keratin hair treatment" actually uses an alkaline chemical to break the disulfide bonds of curly hair and effectively temporarily straighten it. The "keratin" part of the treatment is actually the last part, that which conditions and strengthens the hair as well as reversing damage by the straightening chemical. Your hair still get wet and washed by shampoo.