When Montag gets out of the river, why are his senses so acute?
I agree with the first post. This is the first time that Montag is thinking - actively thinking. Prior to this awakening initiated by Clarisse and then Faber and Beatty, he was passive, a zombie really. During the events that led up to the fire, Montag struggled with his new perspective on the world and tried to cope with either returning to his old life as a fireman or shedding that life. Of course, he eventually does this. But even during the burning of his house, he still is in in between (mentally) his old and new way of thinking. It isn't until he's floating (or out) in the river that he is committed to thinking completely for himself: this moment of clarity or epiphany comes at the point he gets out of the river. His senses are so acute because he is thinking, and thinking clearly, probably for the first time in his life.
I think that there are at least a couple of reasons why Montag's senses are so acute in this situation.
First, it is often said that your senses get more acute when you are in a dangerous situation. I do not know if it is true or if you just feel that way. At any rate, he is in a very dangerous situation and will therefore be more aware of things.
Second (and this seems more important to me) he is now really alive for the first time in a very long time. He has crossed the river and has been reborn in the country. This is a place that is so different from the city and its society. So he is experiencing all these things for the first time. He has gotten out of the stifling society he wants to destroy and is now free.