Look into installing the depth sounder inside the hull. You save cutting a hole for it, and if installed properly they work just as good. For example:
Depth Sounder In-hull Transducer Mounting
SJ23 Tech Tip E05 - Depth Sounder Installation, Shoot Though the Hull
7752--Installing an Insids-Hull Mounted Transducer for Depth Sounders
It won't work if you have a cored hull where you want to put the transducer, but even cored hull sailboats will have solid glass as you get towards the keel.
Cheers,
Tim