Dont silicone it. use epoxy resin as silicone wont give you a good signal as it absorbs the sound waves.
well thats theory. And its what the manuals say but:
I dont do theory !!
From far too many years of time spent boating globally.
I'm giving you the real world experience.
It works absolutely fine. Less messy and much more easily removed esp if you make a !*!* up of it and leave an air bubble in or put it in what you find subsequently is an inconvenient place. Or you want to sell the boat and keep the sounder. Epoxy it and its there for ever.
All our boat angling club boats have been fixed like this for years with no problem, and they use their boats here and in Scotland and Ireland relying on the sounders for new fishing marks.
My training boat(s) have all been so fixed and the sounders still work faultlessly at full speed.