The transducer I used suggested a good silicone and that is what I used. I did not do any special prep, other than verify that the sump was clean and dry. This transducer (kit) had a base that you fasten (silicone) to the hull. It has an angle cut to the base, such that you can twist to factor out the vee in the hull. My 218 seemed to only have about 10 degree back there, and honestly even if it is a little off, probably will not affect the operation anyway. I consider the depth to be usefully information, not critical for me. After the base is installed, you add a little glycol liquid, and then insert the transducer.
( Note: I used to have a 198 V series. I cleaned the sump, put a little pile of silicone down, pressed the transducer into the silicone. Since the bilge pump always leaves a little water, the transducer was always in a puddle of water and seemed to work just fine.)
2012 218 DLV - 140 Suzuki
Tybee Island, Ga.