PiranhaMax question

[Wonk]

I purchased a Humingbird Piranhamax 215 to put on my new Hobie Outback. I did the "test" as suggested in the install manual by firing up the FF with the transducer outside the boat over a known bottom to get a base reading. I then used petroleum jelly inside the hull to check if the place I wanted to permanently mount the transducer would give the same reading. I did this testing in my pool and would float the yak over pool toys on the bottom to simulate a little structure. Although this was a very shallow water test (4' deep max) it seemed to give good results. I "glued" the transducer to the hull with a healthy application of 100% clear silicon (bathroom/kitchen caulk). When I took the yak to the river, it gives inconsistent readings as in going from a depth reading (say 18') to 0 feet, bouncing back and forth.

I also had the sensitivity set to maximum to get a "picture" of the bottom. If set at minimum it would not show a bottom line.

Have I screwed up the transducer install or possibly a bad location or is the hull material or the silicon my problem?? Any help would be apreciated.