Instructions
Play a duet with Carben, the Guardian of the Ocean Realm, to receive a map to the Ocean Temple just as you received maps to earlier temples. Follow the map toward the temple; you'll have to activate the three switches along the way.
Travel to the southeastern corner of the map to find the first switch atop a high pillar. Blow your train whistle to activate it.
Find the second switch in the northern part of the map near a distant rock formation. Blow your train whistle to activate it.
Find the third and final switch at the apparent end of the tracks where they reach a dead end. Activate it with a final blow of your train whistle and the gateway to the Ocean Temple rises from the waves.