There's a reason so many movies are about creepy and haunting things happening at sea. The ocean is deep, dark, cold (RIP Jack Dawson), and full of weird creatures, some of which are bigger than houses and others that literally feast on blood [Jaws theme]. No wonder sailors (allegedly) drink so much!
1.) From CatHerder75:
I spent 7 years sailing a boat around the Caribbean. I sailed alone 90% of the time. Not too many weird things to be honest.
Whales got too close for comfort a few times.
I had auditory hallucinations several times due to sleep deprivation (downside to sailing solo), that was always a bit scary.
Weirdest was sailing between BVI and St Martin, moonless night, pitch black, total flat calm (flat seas are rare in that area). the ocean lit up with what I could only assume were some sort of glowing jellyfish. These globes in the water would flash bright green for a second, then vanish, hundreds of them. And they were not small, several feet in diameter. This lasted for about 30 minutes. It was beautiful and surreal. Never saw anything before or since like it in over 20,000 miles of open ocean sailing.