The thing is, this is actually a race. If you swim down after reaching the song tablet, you will find another tablet indicating a time. It was 1:30, I think. So you need to complete the course in a limited time. The course is easy to recognise, as it has some checkpoints.
To finish this game, you will need to use speed-boosts and even stop time (enter Spirit form). It's certainly do-able, but you will need some time to get used to it.