I have a Simrad IS15 system as well. The depth sounder is Simrad (actually an Airmar B774VL triducer) mounted through the hull under the forward end of the cabin sole plate outside the head. The triducer (like the wind sensors) is connected to an IS15 Transciever before the displays. It functions very well. On occasion it will not start when the system is powered up, and I have to turn the depthfinder off then back on again to get it started. Its absolute maximum depth is about 150m. The minimum is less than the 5 feet that it takes to float the boat (about 2.5 feet below the sensor). I have twice in 8 years had the water temperature sensor fail. That has been fixed by replacing the insert. The bottom of the triducer must be kept free of growth. Water reducable bottom paint is good for a little less than a year, then it must be scraped with a plastic putty knife.
We have sailed in places where the bottom is soft enough to sail through. You know, when you go over the side and swim down, it is hard to tell where the water stops and the bottom begins... sort of a gradual decrease in fluidity. The depthfinder seems to sort it all out and give as good an indication of the depth as can be managed.
1988 PSC 34
Irish Eyes to the Bahamas