How to make Vanilla NPC's drop a modded item?

I want to make my Weapon called "WaterKnife" drop with a 10% chance from HM Ocean enemies (jellyfish, sharks, crabs).
Does anyone know how I could do that easily?
Thanks.

And also, if anybody could tell me how to make my throwing weapon act like a boomerang (spin, go out, and retract) that would be great too.