you should analyse each sentence of the song and what they say.
i'm reading the lyrics right now and really, they dont tell me much. you can say that, their experience of "riding" in the submarine has inspired them through blue sky and green sea. in a way, they speculate about the wonders of the ocean and deep in it. hmm yeah well to be honest, i dont think theres much on this. maybe try
Lucy in the Sky with Diamonds, also by the beatles