Honestly if you feel it's time, then it's time! I got my dog put down two months ago and although the most heartbreaking thing I've ever had to do, it put him out his misery!
I was in the exact same position as you, I held on to him as long as I could but it's time to think of the dog and not how you feel!
It sounds, and not being a vet he may have kidney problems, especially if he is drinking alot! He will probably be on/or has been on Metacam for his arthritis? This does damage the kidney but it does make the dog alot more comfortable.
When you do decide to get your dog put down, I recommend you go in with him. I didn't think I would have the courage to do it but I would have hated to leave him on his own.
Also, please don't let him pass away in his sleep, let him be with all the people he loves and lived with his life.
I know I can't tell you to get him put down, but if he is in constant pain, not sleeping because he's incontinent, then he would thank you.
Someone posted a nice poem on this site after Rex got put down and my Mum typed it out for me and wrote 'a note from Rex' at the top of it. It took me about a month to actually read it because I knew how much it would hurt but when I did read it, it made me smile because it was like he was reminding me of all the good times we had. It's now in my 'memory box'.
If you do decide to get him put down, get him sedated first, this will make him fall into a very deep sleep and you can have ten minutes with him before they give him the other injection!
Honestly it sounds like it's time to think of the welfare of the dog rather than your feelings! There is going to be no miracle cure and the last thing I would have wanted to happen to Rex was to walk in to the livingroom and him to pass away with nobody around him!
I'm sure you'll make the right decision though!!