Fox, on that sounder for fish, you'll only receive the temp of your hull and bilge water if you install it in the hull. I mounted one off the back of my transom, on the centerline, under the water where not only heel won't affect the measurements, but it stays under and hidden.
t37, what is the matter with your datamarine gauge? what is it doing? I just rebuilt the depth, knots, distance, AWI and speed on my old boat. Perhaps I can help (and save you tons of money in the end!)