Wherever the fat pipe upwards from the fuel tank goes.
On a new boat (whether owning or just delivering it ) you do a lot of "....where does that pipe/wire go to". I think it's near the gunwale on the deck o n the starboard side of the cockpit, but unless it's actually marked DIESEL or GASOIL or FUEL don't put diesel in it before following the pipe to the tank, and checking that tank also has a little thin pipe going to the engine!
