I use a factory wet mount from Garmin. It recommends using mineral oil in the well but I use fresh water and it works fine. I've considered putting saltwater in to see if I get better readings but never got around to it. I get good readings to about 700 ft. I remove the heavy transducer when transporting the yak upside down. This model allows you to install on a surface not level to the bottom and then adjust the transducer so that it is level.
Garmin - Garmin 200/50KHz, 12/45deg, Plastic, Adj. In-hull Mount Transducer 010-10327-00
Installed 2006, still no leaks.
I was sloppy with the 3M 5200, but the bottom of the well is clean so that it is only shooting through water and the yak's plastic.