Hi jaticker,
Welcome to the Forum, typically most smoke for 4 hours or less, so you would only need water in the bowl during the smoking process to extinguish the Pucks, secondly the bowl catches the grease / drippings from the meat being cooked.
yes you will loose some heat from opening the door, which comes to the next point, most people will not open the door very often, like my self I preheat the smoker and smoke generator, for 40 minutes, then place the meat in , the water bowl full of water ,, let the smoke roll , once the smoking process is complete i remove the burnt bisquets and bowl replace with a bowl of orange soda or other flavored drink and continue cooking,,, at this point in time I also insert a remote thermometer into the meat, yes it is digital and monitor the temp of the meat till i get it to the desired temp then pull...
other then the two or three times i have opened the door above i keep it closed,,, only exception is when i do a mop on a piece of meat and that is only done when it is almost done and over the course of the last hour or so