A 20 litre jerry can filled with water will be quite heavy, actually it will be in excess of 20kg!.

To get the water out you either need to tip it up or install a tap, even then you will need to sit in on something and fit a the tap and you no longer will be able to fit in your jerry can holder...have a look what I have done in the photo below...this makes things easier.

Drop into you local caravan parts supplier and get a caravan sink pump, it will cost you about $40. Using a hole saw cut a hole in the jerry cans lid, attach a short hose to the pump, long enough to reach the bottom.