Most Beneteaus of that era had a breaker (either dedicated or spare) for the macerator pump (if you have one) and a momentary switch that required continuous pressure on the switch to operate. The switch is mounted on the side of the lavatory sink cabinet. The shower pump had an on/off switch, that was also on the side of the lavatory, positioned in the on position for the duration of the shower. Note that a macerator was optional equipment at increased cost; therefore, the boat may not have one. No, you definitely don't want to empty the tank by way of removing the white panel. The panel provides access in the event that the entire holding tank needs to be removed. Pump out at a marina pump out facility by way of the pump out deck plate located on deck directly above the holding tank. After pumping out the contents, add a few gallons of water and pump out again, a couple of times to remove as much waste as possible. Hope this helps, I haven't been on a 331 in seven years, so my memory may not be exactly correct.