I may be wrong, but the UTHs I've used all say don't use with water.
I believe it was because the heat buildup where the mat attaches, will crack the glass when it comes in contact with the water.
Do you really need a heat mat? What are the wintertime temps in your room? And I believe a heat lamp will do well...just use a small bulb.
Another idea...glass cages.com sells a heat pad that does'nt get attached. The tank sits on it. I use a couple, and they dont get anywhere as hot as the other brands, only because they dont really have perfect contact with the glass bottom.
And one last idea...Ultratherm UTH, available from the Bean Farm.
I dont believe they get as hot either.