The gasket on your fuel cap has probably perished or the cap is worn enough it doesn’t sit tight to the neck.
A full sample jar of water is a gotcha as it is easy to think it is a clean fuel sample.
My only comment would be if there is that much water when you think you have all the water out, I would get the tail down and give the wings a good shake, let it sit for a few minutes and I guarantee you will get more water out of the drain. I would also do a good 10 minute idle then a full runup to ensure there is no water in the lines from the tank.