Nanami's father is a fisherman; she helps her mother out when he goes on long fishing trips.
There's a rumor that as a child she was saved from drowning by a large fish, perhaps contributing to her love for them.
Ironically enough, she gets seasick easily and can't swim. Notice the inflatable in her unawakened SSR?
Nanami has a lisp that is mostly prominent when she is speaking quickly and in an excited manner, but she can speak correctly if she concentrates.
She loves to carry around a plush fish named Sabaori!

Nanami's signature is a doodle of a fish, but if you look closely, you can see its fin is the number 7 with a small hiragana for "mi" at the end. It's a clever detail that plays on the fact she has the character for 7 (nana) in her name.
On the topic of her name, it means seven seas!