Well done to think about it and find it, I like the water warm / fuel cold trick.
There was a crash, I think in Spain, some years ago with a piper that'd been outside for a week and the tank cap seals let water in. It wasn't spotted because like yours it filled the whole strainer and the pilot thought it was fuel and didn't run enough fuel off both tanks before taking off.
BTW .. whatever happened to the old Coconut Airways outfit after the owner died ? I flew their Tomahawk a couple of times when on holidays .. indeed I think I got your Archer in my book once about 5 years ago.