If you bed it in a blob of silicone in a area where the hull is solid it will work fine down to hundreds of feet (100+). You will lose some definition if it's a fishfinder but I did that with my Lowrance HDS7 C-P/Radar/high-definition fishfinder and it works just fine for my needs. It does show fish and schools of fish, along with bottom structure and weed/grass/mud. Maybe not as well as it would if mounted "in the clear", but I'm happy with it.

Also I don't care much about the depth in 100+ feet, and the boat came with a regular Raymarine ST60 speed/depth/wind package so I have the digital display for that (and yes it does co-exist with the fishfinder, which is dual-frequency).

One other caveat: Try to make sure your silicone blob does not have any trapped air bubbles between the transducer and the hull.