A lot of boats that are cored, the coring ends at the waterline. Even in boats that extend the coring below the waterline there is usually a solid section near the midline.
This is a thick area of fiberglass
and the depth
performance does tend to suffer with reduced range and more anomalous readings, but does not require a hole in the hull.
A simple test is to fill a plastic bag with water
seal it (no air). Place the bag on the hull and the transducer on top (the transducer is not in the bag just on top). If the depth
does not work try another spot. Even solid structures have some air voids.
If the works OK you can mount the transducer permanently in the same spot. There are various ways, but a PVC pipe attached to inside of the hull an filled with oil
is the most common.