Choro Q Marine Q Boat

Select same boat

Select "Free Run" mode, then press Up(2), Down, Up, X at the track selection screen.

Extra mode

Complete all the missions, finish in first place on all tracks, and collect all stone signs from the tracks. The "????" selection on the options screen will turn into "Extra mode". Use this option to access a "Death Fight" feature and the bonus Marinoid Bay track.