C-25? Trailer?
Yes, don’t get a wrench near that through hull until the boat is out of the water, a small trickle can become a huge hole in seconds flat if that plastic transducer decides to break in two. When you re-bed the transducer use something like Life Calk or Sikaflex, it seals well and is easy to remove when it’s time to upgrade. It may not be what's leaking though! The transducer holder is like a mushroom with a threaded stalk with a hole in the center that penetrates the hull through a large-ish hole. The transducer is calked then pushed through the hull then a nut is threaded down and the whole mess cinched up. May that part never leak again! Most speed transducers (okay then; all I have seen) can be removed from inside the boat to remove marine growth. Usually the center of the transducer can be removed by pulling a pin, then removing the transducer by pulling up. Once the transducer is pulled out you will need "the" or "a" plug to replace the transducer pronto to quell the inrush of water. The transducer will have a couple of O-rings that keep the sea at bay, and most likely these are worn and need to be replaced. If you have "the plug" you should be able to do all this right at your slip. With a C-25 you ought to be able to pull it out with a trailer and boat ramp. You won’t need to unstep the mast, as the repair is quite easy.
