You can use any sealant between the hull and the transducer if the location is suitable and the hull is solid. If the water bag test failed then either the transducer has gone bad or the location is not good. Keep trying other locations. On my H31 I have two silicone mounted transducers, one Garmin, one Raymarine both work with no problem. The Garmin is 500W, goes to more than 1,300 feet, the Raymarine is 120W, and less sensitive.