Re: What are these things?
I'm only guessing, but sometimes previous owners leave the old transducers in place when they replace old and failed instruments as an easy option. The transducer is simply to provide either depth or speed information to an instrument (sometimes both combined) so has nothing to do with the bilge. If you look at the spot from outside the hull, you should see the other side of the transducer protruding through which will be either flat (for depth) or a paddle wheel (for speed).
Most instruments using through hull transducers also supply blanking plugs so the transducer can be removed for servicing. If you search around on your boat, you might find the blanking plug for it if you are lucky.