Yeah, definitely 20L was more than was required. We ended up tipping out a heap of water before leaving.
On average we were using around 2.5 litres per day for drinking per person and about the 3 litres per day for baths per person. We usually will heat our water up on the stove, and recirculate it with a 12v shower.
We were generally doing our washing up with ocean water, but sometimes we did use fresh water for this so another 3-4 litres there.
Yeah, I think 5-6 litres per person per day to be reasonably comfortable is good. Could go to 3-4 litres per person day, if you really had to push the distance between water refills.