That's a nylon-insert locknut ('nylock' in the vernacular), where the nylon on the outside end resists rotation on the bolt threads, but isn't structural. If it were a standard nut, it would be about flush with the end of the bolt. You generally want a couple of bolt threads exposed beyond the nut. You also want stainless steel, and from the look of the rust dribble, I'm guessing it's plated, not SS. If it were me, I wouldn't lose sleep over it, but I would replace it in the fall or spring when it's on the trailer. With a thick plate rather than the little washer, as the rubber is mushing out from under it. Someone with that model boat, and there are a lot of them here, should be able to advise as to the correct sealing, gasket, and washer arrangement.
And ditto K9's observation - that looks like a transducer, probably depth or fish. Don't take it out while the boat's in the water.
