The temperature-dewpoint spread was 6 degrees, outside the normal range for fog. However, it could easily have been foggy at the crash site. Boaters in the area reported poor visibility, and other pilots reported reduced in-flight visibility. Even with visibility of more than three miles, that may not have been enough to maintain visual orientation over the ocean. A moonless night, or one with high cloud cover to block the moon or stars, makes it extremely difficult to distinguish sky from ocean.
Flight instruments become a necessity to stay oriented.
In daylight hours, the odds would have improved significantly. If a localized fog bank formed, it would have been visible and more easily avoided in daylight. The physiology of night flight contributes to its difficulty. Visual illusions under those prevailing conditions are well known to accident investigators. The flying public should understand them as well.