Tides -> oceans -> gravity-> people's moods seems to be based on Argument by Similarity - the idea that just because two things look similar, they must be connected in some deep way.  

But how does being made of water change anything? The tides go up and down. They don't have moods or personalities. They're completely predictable and mechanical.

So where do changes in mood and behaviour come from?

The only connection is a poetic one - moods ebb and flow, the sea ebbs and flows (even though tides are mechanical), so therefore, an obvious link.

But isn't this just taking a metaphor literally?

Being made of water doesn't really make anyone moody, surely?

Do unemotional people have less water in their bodies than moody people?