I don't like to put the NB in the water if there is any other way to get it into the snake (inject into food, dust on food, force feed water and NB with eyedropper, etc). That is because the NB goes bad in the water within 24 hours or so. If you must do it that way, I would take away the water for at least a day so the snake is very thirsty. Then mix the NB is a small amount of water and give it to him. Then throw it all away after 24 hours. It is wasteful, but will work if you can't do it any other way.
Be sure to keep the bottle of NB VERY dry - if you spill any water into it, it will go bad quickly. If you keep it cool and dry, it lasts a long time.