Depends on how sensor is attached to the hull. ideally it shoud be transon mounted, as it was designed for it. But you could give it a try and see how different materials impedance would affect its capability. The easiest way, is put it inside a plastic bag filled with water (oil would be beter, but messy), then put bag in contact with hull (away of keel, as it could interfere on bean if too close), resting sensor faced down to the hull. This way, you're getting reads from some different materials (water, then plastic bag, then GRP hull, then saltwater). If that prove enough, with good reading, then you could glue the sensor inside the hull, using same resin used on lamination. This way you'l.l have just one extra layer (the resin) before bean going to salt water. In other words, if this works on a bag filled with water or oil, it will work beter if glued inside. Search here for this topic and you'll find some interesting experiments.