Removal
I have actually helped in the installation of a thru hull for salt water intake on an AC installation while the boat was wet. Obviously, a lot of planning, extra hands, the right tools, etc., are required. After the hole was made in the boat, a large diameter hose was held over the hole effectively raising the the waterline inside the hose. That kept the owners blood pressure in check. Another hose was snaked through the first, out the hole, and up to the surface, where another set of hands grabbed it, inserted it into the bronze tailpiece, applied the appropriate water setting polysulfide, and the hose was then retracted from inside the boat, pulling the fitting along with it. Inside the hole it arrived, ready for another set of hands to grab it. The wood bedding block and coupling nut were already down the hose with plenty of "gunk". The wood bedding and nut were appropriately secured to the through hull fitting. The rest of the operation should make sense. I have seen this done twice, and am impressed with how well it worked. It takes guts though. In your case, however, their are two problems, one the removal of the old, and since the depth sounder is solid, creating a way to pull the new sounder back through the old hole. The first problem, outside of the sheer force of removing the transducer, is removing the old bedding compound. If that is poorly done, no amount of fresh stuff will keep the job from leaking. On the second, you do have the wire to use, but that would be too loose below the waterline. You can't have the new fitting bouncing around all over depositng the bedding compound everywhere except where it is needed the most. Given this, a haul out and hold in a cradle would make the most sense. You don't need to wash, or block, and a marina should haul and hold while you exchange the fitting very cheaply. And without the worry and anxiety.