Jump scared by the credits page since my PC's been BSODing occasionally the past month.
Given the familiars actual nickname was Sameko in the raw, if you're going to translate it to English it should be 'Sharkie'. Since '-ko' is also used to make cutesy nicknames, especially for pets and gets used to show a character has lazy, immature or bad naming sense, all of which applies to this scenario. And adding '-ie' or '-y' is what many people do in English for similar purposes. A related example would be that the English approximation of wanko would be doggie or woofie. This makes the protagonist dialogue irrelevant, thus would require a TL note, but that's the cost of the liberal translation approach you went with for the name. Personally I think it should have just been left as Sameko. (I know this is trivial to the chapter, it just annoys me seeing weird attempts at mixing Japanese with English)