That's a good point about the underwater physics! At one time, we actually wanted to animate floating hair for everyone, but it turned out to be too difficult. :'D So our current thinking is that the mermaids do use some magic to keep things put and also use heavier things in general. I think it's a great idea to have the MC comment on it! Maybe we can add a funny scene where she has trouble getting stuff to stay down and another mermaid has to help her. xD
About the Sea and Land Paths, your first intuition is correct. The Sea Path is for Mikali and Malik's routes, and Land Path is for the Sorcerer and ???'s routes. However, all the characters can have scenes in both the Sea and Land Paths (for example, Malik's scene and CG at the beginning of the Land Path), so we encourage trying out both. There's also an option to split your time between land and sea that will be unlocked later on, which will open up a new path with more group events.