Snell's window is around 180 degrees so you wouldn't get anything more of the above-water scene, perhaps some extra areas of internal reflections. I have a Meike 3.5mm Fisheye in m4/3 mount that is 220 degrees at least and it wasn't that useful in my Olympus housing with a port made for Olympus own 180 degree (non-circular) Fisheye. The tiny Meike does better out of water though.
Apart from its rarity (although the Fisheye 6/2.8 isn't among the very rarest of Nikkors, despite the widely seen claims to the contrary), the size and sheer bulk of this lens makes it hardly feasible to put in any underwater housing I'm aware of. It certainly would not go into the big dome of my old Awuatica housing, for example.