I did coke a couple times in college but have never seen or smoked crack. My guess though is that its coke. Running the water and the fan could be to cover up the sound of the snorting. You could cough after snorting coke because it goes down the back of your throat. Also I think I remember it making me thirsty (hense the water). You can get irritable and probably low libido from it too.
My understanding is that crack does not smell, but the act of smoking it (the device you use, etc) makes it smell of "sweet burning rubber" as others have described. Also I think when someone goes on a crack binge they can leave for as long as like 3 days and then come home and sleep.
I think X-dboy meant that he could be shooting up heroin, not coke or crack. I believe there are other places the track marks could be, not just the arms. There is someone on this site who also has experience with this drug that will probably respond tomorrow with her opinion. If I talk to her I will tell you to give you her two cents. Good luck- I hope you have enough self respect to be understanding, but not subject yourself to this for too long if he doesnt want to change.