I have never had any temp issues when I use water but then again I am using a propane rig so if it is too cold I just turn the gas up. I also always pour in boiling water so it is hot right off the start. I just finished my UDS and I was going to experiment with a small water pan in that so I will see if it makes a difference in holding temp or not.
I always put water in the water pan in my smoker but I'm a gasser with a smoke vault 24". Now as far as a rub on salmon I don't personally don't rub any fish I want the fish to speak out for itself. But thats my couple of pennies.