Super Mario Sunshine/Corona Mountain

Entering the Mountain
Once you have every Shine in every world up to level 7, the plaza will be flooded (but not FLUDD-ed). Head to the area where you use the rocket nozzle to get to Pianta Village (there is no point in collecting the rocket nozzle though). There is a cavern that used to be docked up. Go in to enter the volcano.

Crossing the magma river
Hop into the boat. To steer, shoot in the opposite direction to that in which you want to go.