If you are going to Goop, it is best to rough up the area on the yak with some course grit sandpaper so the adhesive has something to grab. Apply the goop and
let it sit for about 5 min to let the solvent flash off. After 5 min, push the ducer in and wiggle side to side to get any big bubbles out. You don't need a lot of Goop in between the ducer and the yak. More goop = more chance for big bubbles. Just a thin layer of it between the kayak and the transducer is sufficient.
Tape it down, let it sit for at least 24 - 48 hours, and you should be good.
IMO guys overthink this, and the tiny bubbles don't affect the reading much. Prior to the through-hull, I Gooped all my transducers down using the method above. They were all a B*&% to remove when I wanted them off, they all had very tiny bubbles, and they all worked very well, even when compared to a spare transducer mounted directly int the water. Even on an old cheapo Eagle fish finder I could track small swim baits to the bottom in 100' of water.
I like the duct putty idea too, but don't like the idea that it can move when transporting the yak upside-down like I do. I can't access where I have my transducer mounted OTW, and I don't wan't to take the chance it won't work once I launch. Keep it simple stupid has always worked...
I guess there are many ways to fillet a WSB
