In general, it is unusual to feel stray current with a 12v system. If the boat is out of the water AND absolutely nothing is connected to land power, I can't explain your sensations.
If the boat is in the water but not connected to shore power, the problem most likely is current in the water itself caused by an electrical leak for another vessel, dock wiring, defective lights etc. It could be anything.
If you boat is connected to shore power, disconnect it and see if the stray current remains. If it doesn't, the problem is in your shore power or your boat's wiring itself.
I am also curious where you were standing when you felt the current touching the engine.