I would refrain from cargo door opening. Damage to the internal mechanism can occur.
@ Truck2005:
The A320 Fam has a vacuum toilet system. So this trick does not work.
What i would do:
First switch off the external aircondition unit.
If you have persons inside which are not unconsciously, tell them to switch on the battery and press the ditching button.
If no one is on board who can act, open the skin air inlet and outlet valve manually.
The aircraft is not completely airtight, so the pressure will be drop even if you do nothing except removing the air source.
If the delta p is lower then 2 psi the fuselage drain valves open and the pressure will drop faster.