A fin is what lets a creature steer through water, so it is read as your ability to navigate feelings. A fin cutting the surface also carries the sense of something approaching unseen.
Has this come up in your dreams before? What was going on in your life then?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.