Hi, I have installed an in-hull (glued) depth sounder (only depth, no fish) inside the bilge compartment at the back of the boat.
I own a zodiac medline IIC (5.3m).
The technician said that place is best because is always under water and it is also a one-piece fiberglass section of the hull. If it is a 2-piece sandwich piece of hull, the eye does not work because of air in-between the hull layers.
I get depth readings even when travelling at 30knots.
Maximum depth before losing readings I have seen is 190m.
I hope that helps.