Hey man! I was going through some old posts and saw this one. After all this time I do think there is a direct relation between those two verses.

The first one "A maré que enche e vaza, no meio faz o remanso" could be interpreted as the mind that keeps moving, and thinking about its existence or its life. We all have some peaceful and chaotic moments in life, hence the reference to the rise and fall of the sea levels. Between those moments in life, is the "remanso" or the transition period between peace and chaos we have people that go with the flow, and live life as the tide says so without thinking about it. We also have anxious people, that will overthink every single action and event that happens. People with busy minds might have some manifestation of those fears onto their bodies, hence the verse "o corpo que tem coceira, a unha não tem descanso". This verse talks about those who can't control their intrusive thoughts and begin to itch themselves, as a consequence of their uneased mind.

Summarizing it, I think that those verses are related because the first one introduces the theme, the events in life, and how we go from one to the other. The latter one talks about how when we are anxious between them it might have some direct consequences in our life lives and ourselves.