Wow... this is unusual. You should use a two part, slow cure epoxy to adhere the transducer to the hull.
The problem is that there was supposed to be epoxy on the location where it was mounted to begin with and if the transducer came unglued, then you can't really re-attatch it on the same spot because you'll likely have air bubbles or an air pocket which would kill the sensitivity of the transducer if you'd be able to get any readings at all.
You'll have to load it in another location on the inside of the hull, being careful to avoid the ribs or the steps on the side of the hull as close to the centerline of the boat as possible.
