Honestly any speed low in the dark over the water is terrifying.
Even with night vision goggles, flying over large bodies of water is like being over a black hole. There is a reason we are required to fly with altitude holds and radar cues in that environment. The hall I used to train in was named after a dude that flew it into the water in the dark.
Old school USN/USCG SAR crews doing night ocean rescues unaided, with only a searchlight and baro altimeter. Hardcore right there….