If you don't know what it is and are not using it and it makes a hole in the bottom of your boat, you may as well remove it and fill the hole.
A picture of the outside of it would be useful. The "fin" part has me curious. My wild-ass guess is an inductive speed probe, likely with temperature but pure conjecture on my part.
Forward of the keel
in relatively undisturbed water
. Using it would require identifying it (brand etc.) which also will likely require pulling it out of the boat and getting a better look at it.
transducer would be nearer the keel
and would be flat on the outside in my opinion.