I've never thought of the fish as Nun - but Ofcourse!
Leading onto the thought - Nun/Naunet is my patron Deity (I'm Wiccan). But that would then be the snake as Nun, and a frog would be the correct representation of Naunet (combined, they are a Sacred Androgyne).
It's entirely possible that this interpretation of the card is a solid one, although I don't know enough about Kemetic mythology to say. However, I'm pretty sure that the "Nun" in discussion here is the Hebrew letter (נ) and its place on the Kabbalistic Tree of Life, not the deity. The letter is written on the bottom of the card, along with the symbol for Scorpio.
A few others...Vision Quest has an evocative take on it. Druid Craft's is graceful and feminine and Illuminati Tarot's view on Death is rather sumptuous.