The levels are usually split up in five parts, and different parts have different "gimmicks". For example, the second part, without spoiling to much involves puzzles with pipes and waterflows.
(Cause last year when I was making it, I was experiencing the same problem of it being repetitive just killing enemies all the time.)
I love the way the song sounds muffled when he swims. I'm guessing you were playing two similar songs in two different channels, with the volume turned down in one of them?
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