Depressingly easy trap to fall into - as the past record described shows - even on what seems like a nice day, easy conditions.
Quite a few years ago I was doing a short delivery flight in a crop sprayer. I was in loose formation with another aircraft, which had flown me to the pick up point and was now returning with me. We were doing a short water crossing heading North. Because the weather was nice we'd flown to cut the corner at the Forth and Tay estuaries, going over water around St Abb's Head. Sunny, calm, light clouds and light hazing in the distance, Quite suddenly I felt I had no reference except on the other aircraft. No horizon - the fuzzy haze, no visible surface because of the calm weather. The presence of a ship in my field of view, making smoke, made things worse, not better. Which way was it going, what was the smoke telling me, etc. Looking across at the distant coastline didn't help either - at what angle should it be? My companion aircraft started to turn gently, back towards the coast which somehow made my disorientation complete. I had to break away, use the limited panel to sort out straight and level and carefully turn back towards land, while descending slowly as straight and level as I could, with my senses telling me I was diving hard to starboard.
A couple of minutes later all was well and I was close enough to the coast to be happy that what I was seeing made sense again. But I felt a total fool, being only a few miles offshore in good visibility and losing the picture so easily. That taught me a lesson about over water flying. Insidious danger.