Directly related to fins:
* Fin: The obvious one!
* Fins: Plural of fin
* Finless: Lacking fins
* Finfold: A fold of fin
* Finlike: Resembling a fin
* Finned: Having fins
* Finfish: A fish with fins
* Finback: A whale with a prominent dorsal fin
* Fin-de-siècle: End of the century, a time marked by artistic and social change
Less directly related but still derived from the root "fin":
* Infinitive: The base form of a verb (this is an older meaning of "fin" related to "end" or "limit")
* Definite: Clear, precise (same root as "infinitive")
* Finish: To bring to an end
* Fine: Of high quality, thin (this meaning is related to "end" but can be confusing since "fine" also has a separate meaning)
Let me know if you'd like more examples!