When cooking tomato sauce for a long time, should you add water?
Tomatoes are mostly water anyway. They're not going to get above boiling point whether or not you add water (or at least not by more than a very few degrees even with salt added - less than the effect of altitude). 

On the other hand the browned bits from the bottom of the pan (often called fond in the US though I've never heard this in the UK and the usage in France is a bit different) do give a nice flavour to the sauce so long as they don't catch. 

If you do add water, you'll need to simmer it down anyway, making that the stirring step still happens, just later. But adding some water fairly early can give you some flexibility. A good time to do this is when it's starting to need constant stirring but you need to get on with the rest of the meal. A low heat and a heavy pan help too.