I play jug but I've never heard of using water to tune it. It's mostly how you shape your lips. Or maybe getting the rest of the band to play in a key to suit your jug. Water would cause problems when you move it around. It would spill. I have a few. The best is a big stoneware beast - probably a gallon - the others are much smaller - bought from a pottery in Crieff - designed to hold whisky but surplus to requirements or faulty or whatever. I got a few odd looks when I tried them out. I imagine people thought that they were for flower arranging or some such nonsense rather than playing.