Steve nailed it pretty good
I have a "gooped" install in my tandem, been in there for 7 years, no problems so far.
I have it thru the scupper in my Trident 15, yes it makes the water drain slower for sure. I drilled the retaining nut and put better gaskets and straps on the rod pod to deal with big surf issues. Great way to go.
On my PA I went with the simple version of a wet mount.
I love it, the image is as good as in the water. I have dropped the transducer over the side to check.
The main benefit of this install is that the cables never come off of the back of the head unit.
Battery goes in a dry bag right next to the transducer, cables are run under the PA hatch. Piece of cake and never exposes those cable or head connections to water.
I made this out of some dense packing foam, and gooped it down, only takes a sponge full of water to get a great signal:
