I am so pleased you caught the theme of water! I always try to tie my fics together with something, and someone had created this picture during the GC where Severus cast Avada Kedavra where he just looked so powerful yet drained. Thought of that image while I was thinking of a story to write.
It's funny that you liked that line best, because I rather did as well. After finally learning to see Harry, I can see Severus being intrigued by Harry being as autonomous as himself.