Looks like a through
hull depth transducer that was glued inside the hull to make it a shoot through. That's not uncommon, I've done it a few times. Shoot throughs are often used to replace a through hull that has failed and they don't want to
haul out to replace it. On closer look it appears the wire from the plastic one has been cut off, indicating the
bronze one is the active unit. If you can get the bronze one unstuck without ruining it, you can remount it in the hole from the plastic one, but if they used
epoxy that may not be successful. If the sounder is working, you're probably better off leaving them alone. If not working remove both and replace the through hull one, maybe the display too if you wish. This might be a good excuse to upgrade to a networked system.