I don't have any experience in dogs with excessive thirst, but do with my son. He was constantly drinking water; even from puddles outside!!
I was horrified and looked for an answer to his problem. To make a long story short, he was diagnosed with NDI, Nephrogenic Diabetes Insipidus, at the age of 2. He dehydrates 4X faster than the average human! It occurs in only 1 in 250,000 human babies! He was such a sight to see whenever we had him in the hospital and doctors would come to see him for real. It is genetic mutation, as a geneticist found out through his dna. Turns out it happened at conception. I am aware that dogs do get this as well. Not sure if this at all helps you with your furrbaby, but thought to post it anyways. There are a lot of people in the USA that have this. There is a website for it - look up NDI and you should find it. Just fyi - it is not life threatening to my son, so his life goes on. He just drinks A LOT of water and should pay close attention to his sodium intake. He is 21 so, as most youngesters, thinks he knows better and doesn't do this. His body has adapted to the large amounts of water he drinks daily and he has a large bladder. If I should have a fire in my house, he will be the first I'll call to help put it out. HAHA!! All joking aside, sending positive thoughts your way in getting to the bottom of what's happening to your Max.