Xducer float
You can make a float form a piece of 2x6 for a standard xducer and tow it with the cable. I would try to shoot through the hull before I rigged anything outside. Glue some foam with a pocket cut into it to the inside of the hull. put some ky jelly into the pocket and set the xducer in it. Make sure there are no air bubbles. If it works your set and if not you can clean it up easily. The foam and pocket also works for mounting rechargable batteries. Cut the pocket tight so it holds the battery tight.
Had the castable wrist watch Humminbird,
it stopped working right after the third trip. The screen is too small bo be of much value for identifying structure. Much prefer my Cuda 128