Have you got a drain well in the boat...a well at the stern where the grp is just single skin i.e. the outside water touches the other side of it
If so take the transducer off its mount, and just put it in the drainwell, you may want to wedge it with a bag of sand on top of it to keep it on place/right way up. Next time you go out chuck some water in the well so that the interface twixt transducer face and bottom of the boat is always under a bit of water.
Try it out.
Will work fine thru one layer of glass and hopefully will work at any speed. If it all works ok then when you get home dry it all out and simply fix the transducer in with some clear silicon avoiding air bubbles. Two pot epoxy is the official way of sticking it down but we jave always used silicon with no probs and being doing so for donkey's years. Can always get the transducer out again if ya want with silicon.
On the other hand it may just be kn******d but hopefully and probably not eh!