It looks like a sensor.?

this appears to be either a oil or heat sensor. You can most likely determine this by checking to see if your oil or temperture gauges work.

how about checking it with the manual instead of guessing ..

this appears to be either a oil or heat sensor. You can most likely determine this by checking to see if your oil or temperture gauges work.

It's an oil pressure sender/sensor. Tell me what year and model engine, and I can probably provide you with a wiring diagram.  

That goofy hex that it has is for an oil pressure sensor (special socket needed) probably just there plugging up the hole as a lot of boats have car engines put in when the original marine engine grenades 

Could be an oil pressure gauge, but their should be a wire to it. Does your oil pressure gauge work?

Looks like a knock sensor.