asssuming you have a single sheet GRP hull construction don't bother to mount it on the transom - find a nice accesable spot inside the hull where it's not too thick or reinforced and 'stick it there' in a large blob of sikaflex (easy way) epoxy (very good but a pain to move) or it's own personal oil bath (over complex and unnecessary).
the fact that it's a skimmer type transducer won't matter one bit. you will loose a bit of power so don't expect brillant images of fish in 600ft of salt water!
Many manufactures web sites have fuller details.
Over the years I have used silicone, sikaflex, araldite, epoxy resin without noticing any difference between them in performance terms.
to be honest I believe it works fine with any reasonable single skin.
to find a good spot you can use the follwowing trick - fill a plastic bag with water and tie off. Place bag where inside hull where you wish to 'check' and place 'ducer on top of the bag as horizontal as possible. Move around to find good signal - worth switching unit to manual gain/sensitivity to get a comparison.
Let us know how you get on!