10 litres of water off of a measured amount of 50 litres of fuel, seems an awful lost as Ian says.
Obvious answer is.........measure the amount of fuel left in the tank accurately (assuming the 10 litres is an accurate measure of the amount of water removed), if you're left with 40 litres of fuel, you are correct that is is an ethanol issue, if you still have 50 litres of fuel you have a water ingress issue, simples I think!!!
Hth,
Rob